LECTURES 

ON 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

Deceived       MAY    5    1894       t  ,gQ 

Accessions  NoST2^^^  .  Chns  No. 


.ft 


FOUR  LECTURES 


ON 


STATIC  ELECTRIC  INDUCTION. 


J.  E.  H.  GORDON,  B.A., 

•\> 

Assistant  Secretary  of  the  British  Association. 

DELIVERED    AT  THE 

EOYAL   INSTITUTION   OF   GREAT   BEITAIN,   1879. 


Illud  in  his  rebus  non  est  mirabile,  quare, 
omnia  cum  rerum  primordia  sint  in  motu, 
summa  tamen  summa  videatur  stare  quiete, 
praeterquam  siquid  proprio  dat  corpore  motus. 
omnis  enim  longe  nostris  ab  sensibus  infra 
primorum  natura  jacet. 

LUCRETIUS,  II.  308. 


D.    VAN   NOSTRAND,   PUBLISHER, 

23,  WARREN  AND  27,  MURRAY  STREETS. 

rj _^    ^  1881. 

>*^ 


CONTENTS. 


LECTUHE  I. 
THURSDAY,  January  16M,  1879. 

PAGE 

Introductory    .......         1 

Preliminary  Experiments  .....          4 

Conductors  and  Insulators         ....         8 

Equal   quantities   of  the   two    electricities    are 

always  induced .         .         .         .         .         .12 

Statement  of  the  problem,  "  When  induction 
takes  place  between  two  bodies,  what  is  the 
nature  of  the  action  across  the  intervening 

space?" 14 

Induction  is  a  state   of   strain ;   in  conductors 

this  state  is  continually  giving  way    .         .       16 
Mechanical  illustration     .....       16 

When  current  passes,  strain  gives  way,  and  in- 
duction ceases    .         .         .         ...       20 

Strain  of  glass  released  by  heating  it .         .         .       21 
Impossibility   of  producing   one   kind   of  elec- 
tricity only        .         .         .         .         .         .22 


IV  CONTENTS. 

LECTURE   II. 
THTJBSDAY,  January  23rd. 

PAGE 

Continuation  of  arguments  for  supposing  induc- 
tion to  be  a  state  of  strain. 
The  Leyden  jar         .....         .25 

Residual  charge 29 

Mechanical  experiment  to  explain  it  .  30 

Phenomena  caused  by  straining  of  glass  of  jar     .       31 
Hopkinson's   experiment,  showing  that  the  re- 
covery from  the  state  of  strain  is  hastened 
by  mechanical  tapping        ....       34 

How  is  the  electric  strain  propagated  ?  .39 

Faraday's  study  of  induction  in  curved  lines       .       40 
Experiments  showing  that  induction  can  turn  a 
corner,  and  is  therefore  not  a  direct  action 
at  a  distance      ......       40 

Induction  must  precede  discharge      .         .          .       44 
Curved  discharge      ......       45 

Faraday  and  Maxwell's  theory  of  lateral  pressure 

accompanying  tension  force  of  induction      .       47 
De  la  Rue's  observation  of  a  lateral  pressure  in 

vacuum  tubes    .         .  .  48 


LECTURE   III. 
THUKSDAY,  January  3Qt7i. 

If  induction  is  a  state  of  strain  of  the  medium 
through  which   it   is   propagated,  different 


CONTENTS.  V 

PAGE 

media    should   propagate   it  with   different 
strengths  ;  that  is,  should  show  differences  of 
specific  inductive  capacity.     Faraday's  ex- 
periments show  that  they  do  so .          .          .  49 
Faraday's  measurements   .....  53 

His  results       .......  60 

Keasons  for  accurate  measurements    .         .         .61 

Recent  measurements  by  the  Lecturer        .         .  63 

Theory  of  the  new  experiments .         .         .  65       £  ij 
Experimental  details  :— 

The  induction  balance         ....  73 

The  coil .79 

The  rapid  break          .         .         .         ...  80 

The  secondary  reversing  engine  ...  83 

Plan  of  the  laboratory        .          .         .  84 

Experiment  made  before  the  audience         .         .  86 

Table  of  results  90 


LECTURE  IV. 
THUBSDAY,  February  Gth. 

Specific  inductive  capacities  of  gases  : — 

Faraday's  experiments        .          .         .          .92 

Ayrton  and  Perry's  experiments          .          .  93 

The  open  condenser   .....  95 

The  closed  condenser          ....  95 

Table  of  results          .....  99 

Clerk  Maxwell's  electro-magnetic  theory  of  light  100 


yi  CONTENTS. 

PAGE 

Arguments  for  supposing  electric  induction  and 

light  to  be  strains  of  the  same  ether         .     105 
In  both  cases  the  energy  is  partly  potential 

and  partly  kinetic          .         .         .         .107 
Vibrations   of  electric    induction    are   like 
those   of  light   at    right  angles   to   the 
direction  of  the  ray         .          .         .         .110 
The  theory  accounts  for  the  fact  that  con- 
ductors are  opaque          .         .         .         .113 
Comparison  of  velocity  of  light  with  that 
of  electro-magnetic  induction  .         .         .114 

Table  of  velocities 118 

In  air  and  vacuum  velocities  sensibly  equal     120 
Determination  of  velocities  in  other  media : — 

Of  light 120 

Of  electro-magnetic  induction      .         .     122 
Comparison       .         .         .         .         .         .123 

Experiments    showing  actions  of  electricity  on 

light  and  vice  versa         .         .         .         .126 

Electro-magnetic  action  on  polarized  light 

discovered  by  Faraday  .         .          .          .     127 

Elecfefo-static    action    on    polarized    light 
discovered  by  Dr.  Kerr  ....     133 

Effect  of  light  on  selenium : — 

Increases  the  conductivity  .         .     139 

Produces  a  current     .      -^         .         .140 

Conclusion  140 


tJHIVBRSITY 


FOUR  LECTURES 


ON 

ELECTROSTATIC      INDUCTION, 

DELIYEEED  AT  THE  EOYAL  INSTITUTION, 

JAN.   16,  23,  30,  AND  FEB.  6, 

1879, 

BY  J.  E.  H.  GORDON,  B.A. 

(Assistant  Secretary  of  the  British  Association). 


LECTURE     I. 

JAN.  16. 

INTRODUCTORY. 

"  Amongst  the  actions  of  different  kinds  into  which 
electricity  has  conventionally  been  subdivided,  there  is, 
I  think,  none  which  excels,  or  even  equals,  in  impor- 
tance that  called  Induction.  It  is  of  the  most  general 
influence  in  electrical  phenomena,  appearing  to  be 
concerned  in  every  one  of  them,  and  has  in  reality  the 
character  of  a  first,  essential,  and  fundamental  principle. 
Its  comprehension  is  so  important  that  I  think  we 
cannot  proceed  much  further  in  the  investigation  of 


2  FOUR    LECTURES 

the  laws  of  electricity  without  a  more  thorough  under- 
standing of  its  nature.  How  otherwise  can  we  hope 
to  comprehend  the  harmony  and  even  unity  of  action 
which  doubtless  governs  electrical  excitement  by  friction, 
by  chemical  means,  by  heat,  by  magnetic  influence,  by 
evaporation,  and  even  by  the  living  being  1 " 

So,  forty-two  years  ago,  wrote  the  Master 
whose  memory  is  honoured  wherever  the 
study  of  natural  laws  is  loved,  and  whom 
in  this  place  we  should  more  especially 
remember,  as  the  Royal  Institution  was 
his  home  and  workshop  during  all  the  best 
years  of  his  life.  Need  I  add  that  the 
passage  I  have  just  read  is  from  the  "  Ex- 
perimental Researches  JJ  of  Faraday  ? 

The  subject  of  our  study  to-day  and  in 
the  other  lectures  of  this  course  will  be 
such  of  the  laws  of  induction  as  are  now 
clearly  known.  I  shall  first  endeavour  to 
show  you  what  the  term  induction  means, 
and  what  is  the  problem  about  it  which  for 
fifty  years  students  of  nature  have  been 
trying  to  solve.  The  problem  is  partly 


ON   ELECTEOSTATIC   INDUCTION.  3 

solved  now,  but  much  remains  to  be  done. 
It  will,  I  think,  be  pleasant  to  follow  the 
stages  of  discovery  up  to  to-day,  and  per- 
haps to  look  a  little  forward  and  try  to 
see  what  the  discoveries  which  may  some 
day  be  made  may  lead  to. 

First,  we  have  the  simple  phenomena  of 
induction  and  electrification,  some  of  which 
I  shall  shortly  show  to  you. 

The  unsolved  question,  "  What  is  elec- 
tricity ?"  we  shall  not  attempt  to  touch 
upon.  It  is  sufficient  for  our  purpose  to 
know  that  when  a  body  exhibits  certain 
properties  it  is  said  to  be  electrified,  or  to 
be  in  a  state  of  electrification.  "We  also 
know  how  to  produce  this  state  at  will,  but 
we  know  next  to  nothing  of  its  nature. 
We  do  not  know  whether  the  properties 
of  an  electrified  body  are  caused  by  one  or 
two  electric  fluids  entering  it  or  leaving  it, 
as  water  into  a  sponge ;  or  by  a  motion  of 
its  molecules,  as  when  a  body  is  heated ; 
B  2 


4  FOUR   LECTURES 

or  by  a  strain  or  twist  of  its  structure,  as 
when  steel  is  magnetized. 

We  have  no  conception  of  electricity 
apart  from  the  electrified  body ;  we  have 
no  experience  of  its  independent  existence. 
Let  us  now  begin  the  study  of  certain 
phenomena  of  electrification  which  it  is 
necessary  for  us  to  understand  before 
commencing  the  study  of  induction. 

If  we  rub  a  piece  of  sealing-wax  or  glass 
with  a  silk  handkerchief  we  find  that  it  has 
the  power  of  attracting  light  bodies,  as  you 
see.  The  glass  or  sealing-wax,  after  being 
rubbed,  is  found  to  be  in  the  state  called 
"  electrification." 

I  must  ask  your  pardon  for  repeating  so 
well  known  an  experiment  as  this,  but  my 
reason  for  doing  so  is  that  I  wish  to  call 
your  attention  to  a  feature  in  it  which 
usually  receives  but  small  attention.  The 
point  I  want  you  to  notice  is  that  when  I 
hold  the  electrified  body  near  these  light 


ON   ELECTROSTATIC   INDUCTION.  5 

paper  shavings,  the  action  takes  place  from 
the  sealing-wax  across  the  intermediate  air. 
What  is  the  nature  of  the  action  ?  We  do 
not  fully  know  yet,  but  it  is  called  Induc- 
tion. 

This  is  only  one  form  of  induction,  but  I  *• 
shall  hope  to  show  you  others.  Mean- K 
while,  note  the  following  definition  to  begin 
with. 

Every  electrified  body  from  which  no 
electrification  is  allowed  to  escape  has  a 
particular  action  on  all  neighbouring  bodies, 
and  this  action  is  called  induction. 

Before  we  go  any  further  in  the  study  of 
induction  we  must  inquire,  Is  there  any 
difference  between  the  electrification  pro- 
duced by  rubbing  sealing-wax  and  that 
produced  by  rubbing  glass  ?  We  may 
answer  at  once  that  there  is,  and  the  differ- 
ence is  a  very  curious  one,  namely,  that 
the  properties  of  the  two  kinds  of  electrifi- 
cation are  exactly  opposite  to  one  another. 


0  FOUR   LECTUEES 

By  opposite,  I  mean  this  :  if  by  any  means 
equal  quantities  of  the  two  electrifications 
be  added  together,  they  will  exactly  neu- 
tralize each  other,  or,  in  other  words, 
adding  a  quantity  of  one  kind  of  electricity 
is  the  same  as  taking  away  an  equal  quan- 
tity of  the  other.  There  are  a  great  many 
ways  of  producing  electrification,  but  all 
electrification  is  of  one  or  the  other  kind — 
either  that  of  glass  or  that  of  sealing-wax. 
For  convenience,  glass  electricity  is  called 
positive,  sealing-wax  electricity,  negative. 
Here  I  have  an  electric  machine,  which  is 
simply  a  convenient  arrangement  for  rub- 
bing glass  and  silk  together. 

I  have  shown  you  one  form  of  induction, 
namely,  the  attraction  of  light  bodies  by 
an  electrified  body.  Let  us  now  examine 
the  effect  of  electrified  bodies  on  each  other. 
Here  (Fig.  1)  I  have  some  pieces  of  sealing- 
wax  and  glass,  and  a  means  of  suspending 
any  one  of  them.  We  find  that  sealing- 


ON   ELECTROSTATIC    INDUCTION.  < 

wax  repels  sealing-wax,  glass  repels  glass, 
glass  and  sealing-wax  attract  each  other, 
or,  generally,  like  electricities  repel,  unlike 
attract.  Hence  you  see  that  there  is 
always  a  force  acting  between  electrified 

FIG    I 


bodies,  and  that,  when  the  electrifications 
are  alike,  it  is  a  repulsive  force,  when  dif- 
ferent, it  is  attractive. 

This  force  acts  through  the  air  or  oilier 
substance  between  the  electrified  bodies,  as 
you  see  when  I  put  this  plate  of  paraffin- 
wax  between  the  suspended  rod  and  the 
one  in  my  hand.  It  is,  therefore,  a  form 
of  induction.  What  is  the  machinery  that 


8  FOUR   LECTURES 

conveys  this  force  across  the  air  or  paraffin? 
This  is  a  question  to  which  we  shall  attempt 
to  give  a  partial  answer  later  on. 

Conductors  and  Insulators. — In  certain 
substances,  such  as  metals,  electrification 
is  able  to  move  freely ;  that  is,  if  one  end 
of  a  metal  rod  receives  electrification,  the 
electrification  is  at  once  conducted  to  every 
part  of  it,  as  you  may  here  see.  (Fig  2.) 


nee 


Jtr 


.APROOF 
D\JFLAKE 


The  proof  plane  being  applied  to  the 
end  furthest  from  the  machine,  it  is  found 
to  be  electrified.  These  substances  are 
called  conductors.  In  other  substances 
electricity  is  not  able  to  move  freely,  and 
if  one  end  is  electrified,  the  other  remains 
in  an  unelectrified  state,  as  may  be  seen  by 
substituting  a  glass  rod  for  the  metal  one 
in  the  preceding  experiment.  The  electricity 


ON   ELECTROSTATIC    INDUCTION.  9 

being  insulated  at  the  first  end,  these  sub- 
stances are  called  Insulators.  It  is  possible 
to  insulate  a  conductor  by  placing  it  on  a 
glass  stand,  and  we  can  then  study  the 
movements  of  the  electricity  in  it  without 
the  latter  being  able  to  escape. 

We  are  now  in  a  position  to  study  the 
effect  of  the  induction  of  an  electrified  body 
upon  a  conductor  near  it.  Here  is  our  insu- 


lated conductor  L,  and  we  will  place  it  near 
the  charged  knob  K  of  the  electric  machine. 
(Fig.  3.)  The  jumping  up  of  thesepaper  slips 
shows  that  the  metal  is  electrified,  but  no 
electricity  has  passed  to  it,  and  it  is  electri- 
fied by  induction.  Thus  we  learn  that  by  in- 
duction an  electrified  body  electrifies  bodies 
in  its  neighbourhood.  This  metal  rod  is 
electrified,  although  no  electricity  has  been 


10  FOUR   LECTURES 

transferred  to  it.  Let  us  now  examine 
what  difference  there  is  between  the  electri- 
fication produced  in  this  rod  by  conduction, 
or  direct  transfer  of  electricity,  and  by 
induction.  First  let  us  electrify  the  rod  by 
conduction,  and  let  us,  after  stopping  the 
machine,  test  both  ends  with  the  proof 
plane.  We  see  that  both  ends  are  posi- 
tively electrified.  We  have  stopped  and 
discharged  the  machine,  but  still  the  elec- 
tricity remains  in  the  cylinder,  and  will 
remain  there  until  some  conducting  path  is 
opened  for  it,  as  by  touching  it  with  the 
finger. 

FIG  4 


c 


The  actions  are  just  as  if  a  portion  of 
an  electric  fluid  had  been  forced  from 
the  machine  to  the  cylinder,  had  dis- 
tributed itself  all  over  the  latter,  as  water 


ON   ELECTEOSTATIG   INDUCTION.  11 

finds  its  own  level,  and  had  now  been 
drawn  off  through  my  body  as  water 
through  a  pipe. 

Let  us  now  return  and  examine  the 
parallel  induction  phenomena.  We  see  by 
the  attraction  of  the  papers  that  the  cylinder 
is  electrified  at  both  ends.  Let  us  now  ex- 
amine what  kind  of  electricity  there  is  at 
each  end.  We  find,  first,  that  the  end 
nearest  to  the  machine  is  negative,  the  far 
end  positive.  We  now  see  why  electrified 
bodies  attract  light  objects;  they  first  induce 
on  the  side  nearest  to  them  an  electrification 
opposite  to  their  own  and  then  attract  it. 

Let  us  now  stop  the  machine.  All  signs 
of  electrification  disappear.  Thus  we  see, 
when  electrification  is  produced  by  induc- 
tion, there  is  nothing  analogous  to  the 
transfer  of  fluid  from  the  machine  to  the 
cylinder.  It  is  more  as  if,  to  use  a  bold 
simile,  by  some  straining  force,  the  cylinder 
was  distorted  into  an  electrified  state,  and. 


12  FOUR   LECTCBES 

just  as  when  we  distort  a  plastic  substance, 
any  increase  of  length  is  accompanied  by  a 


FIG  s 


corresponding  decrease  of  thickness,  so  that 
the  volume  remains  the  same,  so,  when  we 
electrically  distort  this  cylinder  by  induc- 
tion, every  increase  of  electrification  at  one 
part  of  it,  that  is,  any  appearance  of  posi- 
tive electricity,  is  accompanied  by  a  decrease 
of  electrification  at  another  part,  that  is,  an 
appearance  of  negative  electricity. 

The  most  rigorous  and  accurate  experi- 
ments have  shown  that  these  two  quanti- 
ties, viz.,  the  increase  and  decrease  of 
electrification  in  a  body,  when  acted  on  in- 
ductively, are  exactly  equal.  I  can  show  you 
a  rough  experiment  to  illustrate  this  point. 

I  have  here  two  gold  leaf  electroscopes, 


ON   ELECTROSTATIC   INDUCTION.  13 

•which  are  exceedingly  delicate  machines,  for 
detecting  small  quantities  of  electricity.  We 
now  again  electrify  the  cylinder  by  induction, 
and  by  means  of  the  proof  plane  transfer 

FIG, 6 


a  little  electricity  from  each  end  to  the 
electroscopes  respectively.  We  now  stop 
the  machine  and  remove  the  cylinder.  Each 
electroscope  remains  charged,  one  posi- 
tively and  one  negatively,  and  each  with  a 


F1C.7 

charge  whose  strength  is  proportional  to 
the  induced  charges  on  the  two  ends  of  the 
cylinders  respectively.  If  these  charges 


14  FOUR   LECTURES 

are  equal,  they  should  neutralize  each  other 
when  I  connect  the  electroscopes  (Fig.  7), 
and  you  see  they  do  so.* 

The  reason  why  it  is  so  important  that 
we  should  see  clearly  that  equal  quantities 
of  both  kinds  of  electrification  are  always 
produced  by  induction  is  that  this  experi- 
mental fact  shows  us  that  the  action  of 
induction  is  to  produce  something  analogous 
to  a  distortion  of  the  electrified  body,  and 
that,  if  this  were  not  the  case,  but  a  greater 
quantity  of  one  kind  of  electricity  than 
another  was  induced,  it  would  show  that 
something  had  been  added  to  or  taken  from 
the  induced  body,  and  the  action  would  be 
more  analogous  to  a  change  of  bulk  than  to 
a  distortion  of  molecular  shape. 

The  problem,  then,  that  we  have  before 
us  is :  "  Given  the  known  experimental 
facts  which  we  have  just  been  considering ; 

*  The  difference  of  distribution  at  the  two  ends  is 
not  sufficient  to  affect  this  experiment. 


ON   ELECTROSTATIC    INDUCTION.  15 

given  that  there  is  an  action,  which  we 
call  induction,  across  air  and  other  in- 
sulators from  an  electrified  body  to  other 
bodies  in  the  neighbourhood  ;  that  the 
induction  causes  these  attractions,  and 
repulsions,  and  *  inducings  '  of  electrifica- 
tions which  we  have  spoken  of,  what  is  the 
machinery  by  means  of  which  this  induc- 
tion acts?  What  is  the  nature  of  the 
lever,  the  rope,  or  the  pushing  pole,  which 
strains,  and  pulls,  and  pushes  across  the 
air,  or  glass,  or  other  non-conductor  which 
we  place  between  the  induced  and  inducing 
bodies  ?"  We  must  attempt  to  answer 
this  question  bit  by  bit,  and  our  first 
attempt  shall  be  based  on  the  difference 
between  Induction  and  Conduction. 

We  have  seen  that  when  a  piece  of  glass 
or  other  insulator  is  placed  in  contact  with 
the  conductor  of  an  electric  machine,  it 
is  thrown  into  a  state  of  strain  and  dis- 
tortion, but  that  the  electricity  does  not 


16  FOUR   LECTURES 

escape  through  it.  When,  however,  a 
metal  or  other  conductor  takes  the  place 
of  the  glass,  there  is  no  appearance  of  such 
a  state  of  strain  at  all.  What  is  the  ex- 
planation of  this  ?  It  is  this.  Equally  in 
conductors  and  insulators  a  state  of  strain 
occurs,  but  in  conductors  this  state  of  strain 
is  continually  giving  way,  while  in  insulators 
it  does  not  do  so.  To  keep  up  the  state 
of  strain  in  a  conductor  would  be  as  diffi- 
cult as  to  keep  up  a  pressure  of  steam  in  a 
boiler  with  a  large  hole  in  it. 

Let  me  show  you  a  mechanical  experi- 
ment in  illustration — only  in  illustration, 
remember,  not  in  explanation — of  what  I 
mean.  Here  is  a  vessel,  U,  connected  to 
the  water-pipes  at  one  end  and  to  a  pres- 
sure gauge,  S,  at  the  other.  There  is  no 
escape  for  the  water,  it  cannot  flow  or 
move,  and  the  gauge  shows  a  considerable 
pressure.  I  now  turn  the  tap  T,  and  allow 
a  stream  of  water  to  escape.  The  pressure 


ON    ELECTROSTATIC    INDUCTION. 


17 


and  strain  is  relieved  and  the  gauge  falls ; 
that  is,  as  soon  as  the  state  of  constraint 
gives  way  and  the  current  flows,  it  is  seen 
that  the  strain  no  longer  exists.  In  the 


FIG. 8. 


analogous  electrical  case,  bodies  in  which 
the  state  of  constraint  easily  gives  way  do 
not  show  the  phenomena  of  strain  or  in- 
duction, but  allow  the  electricity  to  flow 
freely,  and  these  are  called  conductors ; 
while,  on  the  other  hand,  bodies  which 

c 


18  .FOUti   LECTURES 

have  a  great  power  of  resistance  to  the 
straining  force  can  be  greatly  strained 
without  allowing  a  current  of  electricity  to 
flow.  These  are  called  insulators  or  non- 
conductors. When  such  a  body  is  sub- 
jected to  a  powerful  straining  or  inducing 
electric  force,  it  exhibits  the  phenomena  of 
strain  or  induction  very  strongly.  Let  me 
now  show  you  an  experiment  illustrating 
what  I  have  just  stated. 

You  remember  that  when  we  placed  the 
insulated  cylinder  near  the  machine  the 
induction  which  took  place  charged  the 
near  end  negatively  and  the  far  end  posi- 
tively. In  this  experiment  we  are  only 
concerned  with  the  near  end,  and  we  will 
lengthen  our  cylinder  so  as  to  get  the  far 
end  out  of  our  way.  How  are  we  to  do 
this  ?  This  is  a  large  room,  and  no  doubt 
we  might,  at  some  considerable  trouble  and 
expense,  so  lengthen  the  cylinder  that  we 
could  remove  its  other  end  to  a  distance  of 


ON   ELECTROSTATIC   INDUCTION.  19 

some  20  or  30  feet.  But  we  can  do  better 
than  that.  We  will  make  the  whole  world 
part  of  our  conductor.  The  earth,  owing 
to  the  water  in  it,  is  a  good  conductor. 
We  will  connect  this  wire  from  the  cylinder 


STH 

between  the  machine  and  the  cylinder, 
and  the  state  of  strain  will  commence  as 
soon  as  I  begin  to  work  the  machine,  as 
you  see  by  the  divergence  of  the  gold  leaves 
of  the  electroscope  when  I  take  the  proof 
plane  from  the  cylinder  to  it ;  in  other 
words,  electricity  is  induced  on  our  end  of 
the  conductor. 

c  2 


I 


to  the  water  pipes,  and  now  (Fig.   9)  we 
have  one  end  of  our  conductor  on  the  table    r- 
and  the  other  safely  out  of  our  way  some- 
where in  Australia. 

Now  you  see  there  is  air  in  the  space 

FIC.9. 


20  FOUR   LECTUKES 

This  induced  electricity  will  remain  here 
during  the  action  of  the  machine  as  long 
as  the  air  or  other  insulator  is  between  the 
conductor  and  the  cylinder ;  that  is,  as  long 
as  the  substance  between  the  conductor 
and  the  cylinder  resists  the  straining  force, 
so  long  will  the  state  of  strain  be  kept 
up.  If,  however,  I  connect  them  by  some 
substance  which  offers  an  exceedingly  small 
resistance  to  the  straining  force,  as  this 
metal  bar,  the  state  of  strain  at  once  gives 
way,  and  all  induction  ceases,  and  no 
divergence  of  the  electroscope  can  be 
obtained.  The  electricity  at  the  same  time 
flows  away,  and  distributes  itself  in  the 
earth.  This  experiment  is  somewhat  ana- 
logous to  the  mechanical  one  I  showed  you, 
where  the  strain  was  relieved  by  opening 
a  tap  and  allowing  the  water  to  flow  away. 

The  particles  of  glass  move  more  freely 
over  each  other  when  hot  than  when  cold, 
and  hence  we  should  expect  that  hot  glass 


ON   ELECTROSTATIC   INDUCTION.  21 

would  yield  more  easily  to  a  straining  force 
than  cold  glass  would.  The  following 
experiment  shows  that  this  is  the  case. 
Here  is  a  glass  flask  containing  mercury, 


TO  EARTH    ' 

and  set  in  a  dish  of  mercury.  The  mercury 
inside  is  connected  to  the  electric  machine, 
and  that  outside  to  the  earth.  On  work- 
ing the  machine  it  is  found,  first,  that  no 
electricity  can  escape  through  the  flask ; 
secondly,  that  there  is  a  strong  induced 
charge  on  the  mercury  outside.  Now  let 
the  mercury  be  made  hot.  It  heats  the 
glass,  the  particles  move  more  freely  over 
each  other,  the  glass  yields  to  the  straining 
force,  electricity  escapes  through  it,  and  at 
the  same  time  all  induction  ceases. 


22  FOUR   LECTURES 

We  have,  in  this  lecture,  by  various 
means  produced  electricity,  and  we  have 
produced  sometimes  one  kind,  and  some- 
times the  other.  It  is  important  to  exa- 
mine whether  we  can  actually  produce  one 
kind  alone.  If  this  were  possible,  we  might 
actually  increase  the  quantity  of  electricity 
in  the  world.  Experiment  shows  us  that 
we  cannot  do  this.  For  every  bit  of 
positive  electricity  that  we  produce  we  pro- 
duce an  exactly  equal  quantity  of  negative. 
"We  cannot  make  or  destroy  electricity  ;  we 
can  only  strain  bodies  so  that  their  two 
ends  shall  show  opposite  electrical  pro- 
perties. When  we  rubbed  glass  we  pro- 
duced positive  electricity  on  its  surface. 
Was  not  that  a  creation  of  electricity?  No  ; 
for  an  exactly  equal  quantity  of  negative 
electricity  was  produced  on  the  rubber,  as 
I  can  show  you.  (The  rubber,  on  being 
laid  on  the  electroscope,  caused  a  strong 
divergence  of  the  leaves.)  To  show  that 


ON   ELECTROSTATIC   INDUCTION.  23 

this  negative  is  equal  to  the  positive,  a  very 
simple  experiment  will  suffice.  I  rub  this 
sealing-wax  till,  by  the  cracking,  you  can 
hear  that  it  is  highly  electrified,  but  do  not 
remove  the  rubber  from  it.  You  see  there  is 
no  effect  on  the  electroscope.  The  reason 
is  that  the  action  of  the  positive  on  the 
rubber  exactly  balances  that  of  the  negative 
on  the  sealing-wax. 

In  the  electric  machine  itself  equal  quan- 
tities are  produced,  only  the  rubbers  are 
connected  to  the  earth,  so  the  negative 
escapes,  and  only  the  positive  is  kept. 
Here  is  the  machine  placed  on  an  insulating 
stand,  and  a  wire  from  the  rubber  brings 
the  negative  as  well  as  the  positive  to  the 
conductor.  I  work  the  machine,  and  you 
see  even  the  gold  leaf  electroscope  shows 
no  sign  of  electrification.  This  shows  that 
equal  quantities  are  always  produced.  But 
when  we  rub  sealing-wax  and  silk,  and 
remove  the  silk,  we  have  to  all  appearance 


24  FOUR   LECTUEES. 

negative  alone  in  the  wax<  No  ;  for  the 
instant  the  balancing  positive  is  removed, 
the  negative,  by  induction,  produces  a  fresh 
positive  on  all  surrounding  bodies.  It  does 
so  then,  and  not  till  then,  as  we  may  see 
by  making  one  of  those  neighbouring 
bodies  the  electroscope,  while  the  rubber, 
which  has  been  removed,  induces  negative 
on  bodies  near  it.  No  electrification  of  one 
kind  only  can  be  produced  anywhere.  If 
we  charge  a  balloon,  and  send  it  up  as  high 
as  possible,  it  will  still  induce  an  opposite 
charge,  whose  total  amount  will  be  equal 
to  its  own  charge,  on  whatever  is  nearest 
to  it,  be  it  earth,  clouds,  or  clear  air.  We 
have  no  means  of  knowing  how  much  or 
in  what  way  the  earth  itself,  with  its  atmos- 
phere, is  charged  ;  but  this  we  know,  that, 
whatever  its  charge  may  be,  it  will  induce 
an  exactly  equal  opposite  one  on  the  moon, 
the  sun,  and  even  the  most  distant  stars. 


LECTURE    II. 

JAN.  23. 
THE  LEYDEN  JAR, INDUCTION  IN  CUEVED LINES. 

TO-DAY  we  will  continue  our  inquiry  as  to 
the  reasons  for  supposing  induction  to  be 
a  state  of  strain,  and  we  will  now  attempt 


to  obtain  an  answer  to  this  inquiry  from  a 
study  of  the  various  phenomena  exhibited 
by  the  instrument  known  as  the  Leyden 
jar. 


26  FOUR   LECTURES 

The  Leyden  jar,  in  its  most  common 
form,  consists  of  a  wide-mouthed  bottle, 
coated  inside  and  out  with  tinfoil.  The 
wooden  stopper  supports  a  brass  knob, 
which  communicates,  by  means  of  a  wire 
or  chain,  with  the  inside  coating.  In  order 
that  the  inside  and  outside  coatings  may  be 
well  insulated  from  each  other,  they  do  not 
reach  quite  to  the  top  of  the  jar.  Thus 
the  jar  forms  a  system  of  two  conductors 
(the  tinfoils),  separated  by  a  thin  insulator 
(the  glass).  If  we  connect  the  knob  of  the 
jar  to  the  machine  and  work  the  latter,  we 
can  charge  the  inside  tinfoil,  and,  on  re- 
moving the  machine,  this  tinfoil  will  retain 
its  charge  for  a  considerable  time,  as  is  shown 
by  this  electroscope  Eon  the  knob.  (Fig.ll.) 

This  insulated  electrified  conductor  now 
acts  by  induction  through  the  glass  of  the 
jar,  and  induces  electricity  on  the  outer 
tinfoil  conductor.  As  long  as  the  jar  is 
insulated  there  will  be  negative  electricity 


ON   ELECTROSTATIC   INDUCTION. 


27 


on  the  nearest  portion  of  this  outside  con- 
ductor— that  is,  the  inner  surface  of  the 
outside  tinfoil — and  positive  electricity  on 
the  further  or  outer  surface.  (Fig.  12.) 

Now,  let  us,  as  before,  remove  the  further 
end  of  the  outer  conductor  to  the  other 


Insulated.         Not  Insulated. 
FIG.  12. 


side  of  the  world  by  connecting  the  outer 
coating  to  the  water-pipes ;  we  shall  then 
have  the  whole  of  the  outer  tinfoil  nega- 
tively electrified.  Here,  then,  we  have  our 
two  conductors  oppositely  charged,  acting 
on  each  other  inductively  through  the 
glass. 

Some  idea  of  the  intensity  of  the  strain 
may  be   obtained   by  "  discharging "    the 


28 


FOUR  LECTUEES 


jar,  as  it  is  called.  I  have  here  a  pair  of 
what  is  called  "  discharging  tongs,"  which 
consist  of  a  conveniently- shaped  conductor 
fixed  to  an  insulating  handle.  I  hold  the 
"tongs"  so  that  one  knob  touches  the  outer 
conductor  of  the  Leyden  jar  (Fig.  13),  and 


F1G.13 


then  bring  the  other  knob  of  the  "  tongs  " 
near  the  knob  of  the  jar,  which,  we  re- 
member, is  connected  to  the  inner  coating. 
The  strain  between  the  conductors  is  now 
taking  place  through  two  different  insula- 
tors ;  that  is,  first  through  the  glass  of  the 
jar,  second,  through  the  air  between  the 
two  knobs,  viz.,  the  knob  of  the  jar  and  the 
upper  knob  of  the  tongs.  The  glass  is 


ON   ELECTEOSTATIO   INDUCTION.  29 

strong  enough  to  resist  the  straining  force 
of  such  a  charge  of  electricity  as  the  jar 
now  has.  We  now  bring  the  knobs  nearer 
together.  The  straining  force  across  the 
air  between  them  gets  greater  and  greater, 
and,  at  the  same  time,  as  the  thickness  of 
the  air  diminishes,  its  power  of  resistance, 
or  of  sustaining  the  state  of  strain,  gets 
less  and  less,  till  here,  as  you  see,  the 
air  breaks  and  gives  way,  and  the  elec- 
tricities rush  together  with  a  flash  and  a 
report. 

The  straining  force  of  the  charge  which 
we  gave  to  the  inner  coating  is  removed 
with  the  charge,  and  immediately  after  the 
flash  and  discharge  the  falling  of  the  elec- 
troscope shows  that  there  is  no  electricity 
whatever  on  either  coating  of  the  jar.  But, 
see,  now  the  electricity  seems  to  be  return- 
ing ;  the  slight  motion  of  the  electroscope 
ball  shows  that  a  slight  charge  has  returned 
to  the  inner  coating.  On  applying  the 


30 


FOUR    LECTURES 


discharging  tongs  a  spark  occurs  as  before, 
only  it  is  very  much  feebler,  and  the  jar  is 
now  completely  discharged. 

What  does  this  mean  ?  Where  did  the 
second  charge  of  electricity  come  from  ? 
Let  me  show  you  a  mechanical  experiment. 


nc.14 


which  will  help  us  to  an  explanation.  I 
have  here  a  strip  of  gutta-percha,  of  which 
the  lower  end  is  fixed  to  a  block.  As  it  is 
somewhat  small,  I  will  turn  it  edgeways  to 
the  lime-light,  and  project  the  shadow  of  it 
on  the  screen.  (Fig.  14.)  I  now  bend  it 
down  by  my  finger,  and  suddenly  let  it  go. 


ON   ELECTROSTATIC    INDUCTION.  31 

It  flies  up  nearly,  but  not  quite  to  the  vertical 
position,  rests  an  instant,  and  then  moves 
slowly  on  till  it  is  quite  vertical. 

If  a  spring  index  had  been  applied  to  it, 
it  would  have  been  seen  that  while  pressed 
down  it  exercised  a  strong  upward  pressure. 
At  the  moment  when  it  was  at  rest  a  little 
way  from  the  vertical  it  would  be  exercising 
no  pressure,  and  then  it  would  be  seen  that 
as  it  began  to  again  move  towards  the 
vertical,  it  would  again  exercise  pressure. 
The  gutta-percha  was  strained  or  distorted 
by  the  finger.  When  the  straining  force 
was  removed,  the  strain,  suddenly,  nearly 
disappeared,  but  not  quite.  Then,  in  the 
course  of  the  next  few  minutes,  the  dis- 
appearance of  the  strain  or  distortion  was 
completed  slowly. 

The  electrical  case  is  exactly  analogous. 
The  pressure  of  the  finger  represents  the 
first  charging  of  the  inner  tinfoil ;  the 
straining  of  the  gutta-percha  represents  the 


32  FOUR   LECTURES 

electrical  straining  of  the  glass.  The  pres- 
sure on  the  finger  by  the  strained  india- 
rubber  represents  the  induction  on  the 
outer  conductor.  As  in  the  gutta-percha, 
when  the  straining  force  is  removed,  the 
strain  or  distortion  nearly  disappears,  and 
the  upward  pressure  exercised  by  it  en- 
tirely ceases,  so  in  the  Leyden  jar,  when 
the  inducing  electricity  is  taken  away,  the 
strain  of  the  glass  almost  vanishes,  and  the 
induced  charge  disappears. 

The  strain  or  distortion  of  the  glass, 
however,  has  only  almost,  but  not  entirely, 
disappeared;  and  now  that  there  is  no 
straining  force  interfering,  the  particles  of 
the  glass  move  over  each  other  slowly,  and 
in  the  course  of  a  few  minutes  return  to 
their  normal  state.  But  now,  while  the 
inner  conductor  has  remained  insulated,  a 
change  has  occurred  in  the  electrical  ar- 
rangement of  the  particles  of  glass  adjoining 
it.  The  state  of  strain  has  altered.  They 


ON   ELECTROSTATIC   INDUCTION.  33 

have  changed  from  a  more  to  a  less  dis- 
torted shape. 

Now,  in  the  ordinary  phenomena  of  in- 
duction, what  happens  when  we  alter  the 
state  of  strain  of  an  insulator  by  bringing 
a  charged  body  near  it  ?  Why,  it  induces 
electricity  on  any  adjoining  conductor. 
Similarly  in  the  present  case,  when  the 
elasticity  of  the  glass  brings  it  from  a 
more  to  a  less  strained  state,  and  so  alters 
the  state  of  strain,  a  charge  is  produced 
on  the  insulated  conductor,  and  this  is  the 
residual  charge  which  we  have  been  in- 
quiring about. 

"We  notice  that  this  residual  charge  re- 
turns slowly  and  gradually.  Now,  when  a 
body  is  mechanically  distorted,  and  is  re- 
turning to  its  normal  state  by  virtue  of  its 
elasticity,  anything  which  enables  the  par- 
ticles to  move  more  freely  over  each  other, 
such  as  tapping  or  jarring,  will  hasten  that 
return.  If,  for  instance,  we  have  a  heaped 

D 


34  FOUR   LECTURES 

up  tray  of  sand  slowly  returning  to  its 
normal  unstrained  state  of  being  level 
under  the  action  of  gravitation,  any  tapping 
of  the  tray  will  hasten  the  recovery  from 
the  state  of  strain ;  that  is,  hasten  the 
return  of  the  surface  to  a  level  state  by 
enabling  the  particles  to  slide  more  freely 
over  each  other. 

Now,  if  our  supposition  that  these  in- 
duction phenomena  are  the  effects  of  strain, 
and  that  the  residual  charge  is  the  returning 
of  the  distorted  particles  of  glass  to  their 
normal  state,  is  correct,  any  tapping  or 
jarring  of  the  glass  should  hasten  this 
return ;  that  is,  hasten  the  appearance  of 
the  residual  charge.  In  the  Phil.  Trans. 
for  1876,  Dr.  Hopkinson  has  shown  that 
this  actually  occurs,  and  I  shall  now  hope 
to  repeat  his  experiment  before  you. 

For  this  purpose  we  will  not  be  content 
with  our  electroscope,  but,  as  we  wish  to 
measure  electrification,  we  must  use  an 


ON    ELECTEOSTATIC   INDUCTION. 


35 


electrometer.  The  instrument  here  is  called 
the  quadrant-electrometer,  and  is  the  in- 
vention of  Sir  William  Thomson. 

Here  is  the  instrument  in  the  simple  form 
made  by  Messrs.  Elliott,  and  here  (right-hand 
half  of  Fig.  15)  is  a  diagram  of  the  essential 
parts  of  it.  A  sort  of  brass  pill-box,  sup- 
ported horizontally,  is  cut  into  four  quarters 


nc  rs 


or  quadrants,  each  of  which  is  insulated  from 
the  one  next  it,  but  connected  to  the  one 
diagonally  opposite  to  it.  An  aluminium 
needle,  N  N,  is  suspended,  so  that  it  can 
swing  like  a  compass  needle  inside  the 
pill-box.  The  needle  has  a  strong  positive 
charge.  When  wires  from  the  inner  and 
outer  coatings  of  a  Ley  den  jar  are  con- 
D  2 


36  EOUE    LECTURES 

nected  to  the  wires  a  and  b  respectively, 
so  that  the  unshaded  quadrants  are  positive 
and  the  shaded  ones  negative,  it  will  be 
seen  that  the  action  of  all  four  quadrants 
is  to  turn  the  needle  in  the  direction  of  the 
hands  of  a  watch.* 

The  motion  of  the  index  needle  itself  is 
very  small,  but  attached  to  it  is  a  small 
mirror.  Light  from  this  lime-light  falls 
on  it,  and  is  reflected  on  to  the  screen 
where  you  see  this  spot.  The  least  motion 
of  the  needle  and  mirror  of  course  moves 
the  light-spot  along  the  screen.  The  amount 
of  motion  is  noted  by  means  of  the  scale 
attached  to  the  screen. 

*  The  instrument  can  also  be  used  to  adjust  two 
similar  electrifications  to  equality,  for  if  a  and  b  are 
both  positively  electrified,  the  quadrants  will  tend  to 
turn  the  needle  in  opposite  directions,  and  it  will  go  to 
the  right  or  left  according  to  which  electrification  is 
strongest.  When  we  have  varied  one  of  the  electrifica- 
tions till  there  is  no  deflection,  we  know  that  we  have 
made  them  equal. 


ON   ELECTROSTATIC   INDUCTION.  37 

Our  Leyden  jar  in  this  case  is  made  in 
a  form  somewhat  different  to  that  which 
we  have  been  considering.  The  insulator 
is,  as  before,  a  small  glass  bottle,*  but  the 
conductors  consist  of  strong  sulphuric  acid. 
Some  is  put  inside  the  jar,  and  some  in  a 
glass  dish  in  which  the  jar  is  set.  To 
charge  the  jar  a  platinum  wire  comes  from 
the  electric  machine  to  the  acid  inside  the 
bottle,  while  that  outside  is  connected  to 
earth. 

The  jar  is  charged  for  two  or  three 
minutes  and  then  discharged.  I  now  con- 
nect the  inner  and  outer  coatings  for  about 
a  minute  by  holding  the  wires  from  them 
together  between  the  finger  and  thumb. 
The  wires  are  now  separated  and  connected 
to  the  quadrants  of  the  electrometer,  the 
earth  connection  being  removed.  The 
spot  of  light  showing  the  motion  of  the 

*  A  bottle  about  four  inches  high,  intended  for 
making  a  small  Leyden  jar,  was  used. 


38  FOUR   LECTURES 

needle  at  once  begins  to  move  along  the 
scale  at  the  rate  of  about  three  inches  per 
second,  showing  that  the  residual  charge  is 
coming  slowly  out. 

On  tapping  the  edge  of  the  bottle  briskly 
with  a  piece  of  hard  wood,  the  pace  at 
which  the  light-spot  moves  is  at  once 
trebled,*  showing  that  while  tapping  is 
going  on,  the  residual  charge  returns  much 
more  quickly. 

If  we  wish  to  repeat  the  experiment,  we 
can  discharge  the  electrometer,  and  bring 
the  light-spot  back  to  zero  by  holding  the 
wires  together  for  a  moment  between  the 
finger  and  thumb. 

Thus  we  see  that  any  mechanical  vibra- 
tion communicated  to  the  particles  of  glass 
increases  their  freedom  of  motion  among 
each  other,  and,  therefore,  enables  them  to 
recover  more  quickly  after  they  have  been 

*  This  experiment  was  plainly  seen  by  a  large 
audience. 


ON   ELECTEOSTAT1C   INDUCTION.  39 

thrown  into  a  state  of  strain  by  electric 
induction. 

This  experiment  is  nearly  conclusive,  I 
think,  as  to  induction  being  a  state  of 
strain.  If  the  charge  of  a  jar  were  caused 
by  an  action  at  a  distance,  we  should  have 
to  state  that  part  of  an  action  at  a  distance 
gets  entangled  in  the  glass  and  left  behind, 
and  that  tapping  helps  it  to  escape.  This 
hardly  sounds  probable. 

This  concludes  what  we  have  to  say 
about  the  Ley  den  jar.  We  have  shown 
that  induction  is  a  state  of  strain.  "We 
will  now  begin  to  inquire  into  the  nature 
of  this  strain,  and  try  to  find  out  a  little 
about  how  it  is  propagated  from  place  to 
place. 

Is  it  propagated  simply  in  straight  lines  ? 
does  the  inducing  electricity  stretch  out  an 
arm  through  the  insulator,  and  pull  at  the 
second  conductor  ?  or  does  it  act  only  on 
the  particles  of  the  insulator  which  are 


40  FOUR  LECTURES 

nearest  to  it,  leaving  it  to  them  to  act  on 
the  next  set,  and  so  to  carry  on  the  strain 
from  particle  to  particle  till  it  arrives  at  the 
second  conductor? 

Faraday  asked  himself  the  question,  and 
it  occurred  to  him  that  there  was  a  very 
simple  method  of  arriving  at  an  answer. 
If  the  induction  is  propagated  from  particle 


T 


to  particle  of  the  insulator,  it  can  travel 
along  any  direction  where  there  is  a 
continuous  chain  of  insulating  particles, 
whether  this  chain  forms  a  straight  line  or 
not ;  in  other  words,  it  can  turn  a  corner. 
If  it  were  a  "  direct  action  at  a  distance  " 
(whatever  that  may  mean)  it  could  only 
travel  in  straight  lines. 


ON   ELECTROSTATIC   INDUCTION.  41 

The  following  experiment  is  a  modifica- 
tion of  one  designed  by  Faraday  to  show 
that  induction  can  take  place  in  curved 
lines.  No  induction  can  take  place  through 
a  metal  screen  which  is  connected  to  earth. 
The  simplest  way  to  prove  this  will  be  to 
try  an  experiment.  Here  (Fig.  16)  is  a 
large  metal  screen  connected  to  earth,  and  ~T 


L  =  Light.       E  =  Electroscope.  rgS  * 

M  =  Machine. 

I  place  the  electric  machine  on  one  side 
of  it,  and  the  gold  leaf  electroscope  on 
the  other.  However  strongly  I  work  the 
machine,  there  is  no  divergence  of  the 
leaves.  I  now  (Fig.  17)  take  away  this 
screen  and  put  in  a  smaller  one.  In 
order  that  we  may  be  sure  that  this, 
though  smaller,  is  still  large  enough  to  cut 
off  all  straight  lines  from  every  part  of  the 


42  FOUR   LECTURES 

machine  to  the  electroscope,  I  put  the  lime- 
light as  you  see  (Fig.  17.)  You  see  that 
the  optical  shadow  of  the  electroscope  falls 
entirely  on  the  screen,  and  the  shadow  of 
the  screen  entirely  covers  the  machine- 
On  working  the  machine  the  leaves  diverge 
widely. 

How  did  the  induction  get  to  them  ? 
Our  experiment  with  the  large  screen  shows 
that  it  could  not  have  passed  through  the 
screen ;  that  with  the  lime-light  shows  that 
it  could  not  have  come  in  a  straight  line 
past  the  edge  of  the  small  screen;  and, 
therefore,  we  see  that  it  must  have  come 
in  curved  lines  round  the  edge  of  the  small 
screen. 

This  experiment  shows  a  point  of  differ- 
ence between  the  projection  through  air  of 
light  and  of  electric  induction,  for  while 
the  edge  of  the  optical  shadow  is  almost 
on  the  straight  line  from  the  source  of 
light  through  the  edge  of  the  screen,  the 


ON   ELECTROSTATIC    INDUCTION.  43 

electrical  shadow  does  not  extend  nearly 
so  far,  but  the  induction  curves  considerably 
round  the  edge  of  the  interposing  screen, 
and  extends  in  every  direction,  in  which 
there  is  a  continuous  chain  of  insulating 
matter. 

I  think  the  experiment  which  we  have 
just  tried  will  prove  to  you  the  existence 
of  induction  in  curved  lines.  I  will  now, 


however,  endeavour  actually  to  show  you 
a  curved  line  of  induction.  If  I  connect 
an  electroscope  to  a  knob  placed  near  an 
electric  machine  and  connected  to  earth, 
you  remember  that,  when  we  work  the 
machine  so  that  sparks  pass  slowly,  the 
electroscope  shows  a  strong  induced  charge, 


44  FOUR   LECTURES 

which  increases  to  a  maximum  just  before 
each  spark  and  immediately  after  the  spark 
falls  to  zero.  (Fig.  18.)  I  will  repeat  the 
experiment,  as  we  have  not  seen  it  exactly 
in  this  form. 

This  experiment  shows  that  induction 
precedes  discharge.  All  that  we  know 
about  the  subject  shows  that  this  is  uni- 
versal law,  namely,  that  there  must  always 
be  induction  along  the  whole  path  between 
the  conductors  before  discharge  can  take 
place.  It  is  clear  that  this  law  ought  to  hold, 
for  discharge  is  only  the  sudden  breaking 
down  of  a  state  of  strain,  and  there  can  be 
no  breaking  down  of  strain  except  where 
strain  exists,  and  induction  is  strain. 

The  fact  of  a  spark  passing  along  any 
path  shows  that  induction  was  previously 
taking  place  along  that  path.  It  does  not, 
however,  show  that  the  whole  induction 
was  along  that  particular  path  even  a  very 
small  fraction  of  a  second  before  the  dis- 


ON   ELECTROSTATIC   INDUCTION.  45 

charge.  The  induction  might  have  been, 
and  probably  was,  taking  place  along  many 
paths.  When,  however,  the  insulator  broke 
down  at  the  weakest  point,  and  the  spark 
began  to  pass,  the  whole  of  the  induction 
at  once  transferred  itself  to  the  line  of 
discharge  as  being  the  path  offering  least 
resistance,  and  then  the  breaking  down  and 
relief  of  the  strain  was  completed  along 
that  path. 

Given,  then,    that    induction    precedes 

inc.  If 


discharge,  if  we  see  curved  discharges,  we 
shall  know  that  there  was  previously  curved 
induction.  Let  us  look  at  the  discharge  of 
a  Holtz  machine.  It  consists  of  a  barrel- 
shaped  bundle  of  sparks.  (Fig.  19.)  Here, 
in  fact,  are  the  curved  lines  of  force, 
or  lines  of  induction,  or  lines  of  strain, 


46  FOUR   LECTURES 

produced  in  visible  shape  before  you.  The 
centre  lines  are  straight,  and  the  strongest 
induction  takes  place  along  them ;  but  in- 
duction strong  enough  to  produce  discharge 
takes  place  in  curved  lines  through  all  the 
particles  on  all  sides  of  the  centre  line. 

These  lines  of  force  are  real,  and,  I  may 
almost  say,  tangible  things.  They  can  be 
attracted  and  shaped  by  the  hand  and  other 
conductors.  I  place  my  knuckle  near  the 
lines,  and  they  bend  out  towards  it.  This 
means  that  the  positively  electrified  par- 
ticles of  air  induce  negative  electricity  on 
my  hand,  and  then  the  two  electricities 
attract  each  other,  and  displace  the  whole 
line  of  force.  It  would  be  difficult  to  con- 
ceive the  possibility  of  attracting  an  "  action 
at  a  distance." 

We  have  shown  that  the  induction  is  a 
state  of  strain,  and  we  have  studied  the 
direction  of  the  strain.  "We  now  ask,  What  is 
its  nature?  Faraday  showed  experimentally 


ON   ELECTROSTATIC   INDUCTION.  47 

that  the  lines  of  force  attracted  each  other, 
so  that,  if  a  number  of  them  were  side  by 
side  forming  a  "  bundle,"  their  mutual 
attraction  drew  them  together  as  if  the 
bundle  had  been  tied  up  more  tightly. 

Maxwell  has  since  pointed  out  that  this 
is  what  occurs  whenever  a  rope  is  me- 
chanically stretched.  The  pull  tending  to 
lengthen  the  rope  is  accompanied  by  a 
pressure  tending  to  make  the  rope  thinner. 
To  show  you  this  lateral  pressure,  I  have 

FIG  ,20 


here  an  india-rubber  tube.  When  I  stretch 
it  the  sides  press  on  whatever  is  inside 
it,  as  you  see.  Whenever  a  mechanical 
tension  occurs  it  is  accompanied  by  a 
pressure  at  right  angles  to  it. 

Maxwell  has  shown  mathematically  that 
an  electric  induction  acting  as  a  tension 


48  FOUR   LECTURES. 

along  the  lines  of  force  is  always  accom- 
panied by  an  exactly  equal  pressure  at 
right  angles  to  them.  Electric  induction, 
or  tension,  is  a  tension  of  exactly  the  same 
kind  as  the  tension  of  a  rope,  and  the 
medium  which  can  support  a  certain  in- 
duction force  before  breaking  and  allowing 
a  spark  to  pass  may  be  said  to  have  a 
certain  strength  in  exactly  the  same  sense 
as  a  rope  may  be  said  to  have  a  certain 
strength.  Sir  William  Thomson  has  found 
that  the  electric  strength  of  air  at  ordinary 
temperature  and  pressure  is  9600  grains 
per  square  foot. 

Finally,  Mr.  De  la  Rue  has  actually  seen 
in  one  of  his  vacuum  tubes  a  star  of  light 
showing  a  rain  of  particles  thrown  off  at 
right  angles  to  the  main  discharge. 


LECTURE  III. 

JAN.  30. 
SPECIFIC  INDUCTIVE  CAPACITY. 

IN  the  two  previous  lectures  we  have  seen 
that  Induction  is  transmitted  from  particle 
to  particle  of  dielectrics,  and  that  its  phe- 
nomena are  exhibitions  not  of  some  direct 
action  passing  through  the  insulator,  but 
of  something  actually  existing  in  the  par- 
ticles of  the  insulator  itself ;  that  it  is  in 
some  peculiar  straining  of  these  particles 
that  the  causes  of  the  phenomena  will  be 
found. 

One.  of  the  first  questions  which  now 
presents  itself  is.  Do  all  insulators  on 
which  a  given  inducing  charge  acts  suffer 
an  equal  strain,  and  therefore  exhibit  the 
same  quantity  of  inductive  action  at  the 

E 


50  FOUR    LECTURES 

other  side,  or,  on  the  contrary,  does  the 
same  charge  of  electricity  strain  different 
insulators  differently  and  produce  induced 
changes  of  different  strengths  ;  in  other 
words,  are  there  in  different  insulators  dif- 
ferent capacities  of  receiving  strain  from  a 
given  straining  charge  —  differences  of 
specific  "strainability" — that  is,  differences 
of  Specific  Inductive  Capacity  ? 

Various  experiments,  some  of  which  I 
shall  hope  to  explain  to  you,  satisfied 
Faraday  that  the  latter  is  the  case,  and 
that  different  bodies  have  different  specific 
inductive  capacities. 

First,  however,  let  us  make  it  quite  clear 
what  is  meant  by  the  term  specific  in- 
ductive capacity.  Let  a  certain  charge  of 
electricity  be  acting  inductively  across  air 
upon  a  neighbouring  conductor,  and  let  the 
sizes  of  the  conductors  and  the  distance  be- 
tween them  be  such  that  the  strength  of  the 
charge  induced  on  the  second  conductor  is 


ON   ELECTEOSTATIC    INDUCTION.  51 

equal  to  unity.  Let  the  whole  space  between 
the  conductors  be  now  filled  with  some  other 
insulator.  The  strength  of  the  induced 
charge  will  now  be  no  longer  unity,  but  it 
will  have  some  other  value.  The  number 
which  represents  this  value  is  called  the 
"  specific  inductive  capacity  "  of  the  sub- 
stance between  the  conductors ;  in  other 
words,  the  specific  inductive  capacity  of 
a  substance  is  the  ratio  of  the  inductive 
action  across  it  to  that  across  air.  Air 
being  taken  as  the  standard,  its  specific 
inductive  capacity  is  called  unity. 

"We  will  now  examine  some  of  the  various 
methods  by  which,  from  time  to  time, 
students  of  Nature  have  endeavoured  to 
measure  the  specific  inductive  capacities  of 
various  bodies. 

Here,  at  the  risk  of   being  tedious,  it 

will  be  necessary  for  me  to  go  somewhat 

into  details  of  experiments;    but  it  will, 

perhaps,  not  be  entirely  without  interest 

E  2  ' 


52  FOUR   LECTURES 

for  us  to  look  a  little  behind  the  scenes  of 
the  laboratory,  and  see  the  kind  of  diffi- 
culties which  beset  the  inquirer  the  moment 
when,  instead  of,  for  instance,  saying  to 
himself,  as  a  mathematician  can,  "Let  there 
be  a  perfectly  insulated  charge  of  electricity 
of  given  strength,  at  a  given  distance  from 
a  conductor,"  he  has  to  prepare  himself  to 
say  to  his  instrument-maker,  "  Here  are 
working  drawings  of  an  instrument  which 
is  intended  to  place  a  given  charge  of 
electricity  in  a  given  place;  this  portion 
of  the  instrument  is  to  regulate  the  charge, 
that  portion  to  measure  it,  and  this  other 
portion  to  measure  to  1-1 000th  inch  its 
distance  from  that  conductor. 

Can  we  construct  this,  so  that  every  part 
shall  do  its  work,  and  no  two  parts  shall 
interfere  with  each  other?  Can  we  support  it, 
so  that  it  will  not  shake,  protect  it  from  dust, 
and  yet  contrive  that  neither  the  supports, 
nor  the  cover,  interfere  with  the  induction  ? 


ON    ELECTROSTATIC    INDUCTION.  53 

As  Faraday  was  the  discoverer  of  specific 
inductive  capacity,  we  will  begin  with  his 
experiments,  and,  through  the  kindness  of 
Professor  Tyndall,  I  can  show  you  the  very 
apparatus  he  worked  with.  Here  it  is  (Fig. 
21).  Faraday's  wish  was  to  construct  a 
Leyden  jar,  of  which  the  metallic  coatings 
should  be  fixed,  and  always  in  the  same 
relative  positions,  while  the  insulator  should 
be  movable;  so  that  various  Leyden  jars 
could  be  set  up,  which  should  be  exactly 
alike  in  all  respects,  except  in  the  nature 
of  their  insulator,  which  could  be  made  to 
consist  either  of  air,  glass,  sulphur,  or  any 
other  substance. 

If  such  jars  could  be  constructed,  and  if 
differences  were  observed  in  their  behaviour, 
these  differences  could  only  be  due  to  dif- 
ferences of  induction  through  the  different 
insulators,  or  to  differences  of  specific  in- 
ductive capacity. 

The  apparatus  consists,  as  you  see,  of  a 


54  FOUR   LECTURES 

metal  ball,  which  can  be  surrounded  by  a 
larger  hollow  one.  The  outer  ball  is  made 
in  two  pieces,  so  as  to  allow  the  inner  one 
to  be  placed  inside  it.  There  is  a  space  of 

FIG.  21. 


0*62  inch  between  the  surfaces  of  the  balls. 
The  inner  ball  is  supported  by  an  insulating 
stem  of  shellac  passing  through  a  hole  in 
the  outer  one.  A  wire  which  passes 
up  inside  the  shellac  allows  the  inner 


ON   ELECTROSTATIC    INDUCTION.  55 

ball  to  be  put  in  connection  with  an 
electric  machine,  electrometer,  or  other 
apparatus. 

The  space  between  the  balls  contains 
the  insulator.  It  may  be  air,  as  at  present, 
or  the  whole  or  part  of  the  space  may  be 
filled  with  glass,  sulphur,  &c.  Faraday 
preferred  only  to  fill  half  the  space,  and 
then  to  calculate  what  the  effect  would  have 
been  if  the  whole  space  had  been  filled. 
He,  therefore,  prepared  his  insulators  in 
the  form  of  hemispherical  cups.  Here  are 
some  of  them. 

He  constructed  two  of  these  Leyden  jars, 
so  that  he  could  observe  simultaneously 
their  actions  with  different  insulators,  and 
endeavoured  to  make  them  precisely  alike. 
If  they  had  not  been  precisely  alike,  there 
would  have  been  a  difference  in  their  be- 
haviour which  would  have  been  due,  not  to 
difference  in  the  specific  inductive  capacities 
of  the  insulators,  but  to  differences  in  the 


56  FOUR   LECTURES 

shape  and  size  of  the  jars.  In  order  to 
make  sure  that  they  were  exactly  alike,  he 
made  an  elaborate  series  of  preliminary  ex- 
periments. We  need  not  go  into  all  the 
details  of  these  preliminary  experiments, 
but  we  can  indicate  the  principle  of  them 
in  a  few  words. 

The  object  of  them  is,  we  remember,  to 
determine  whether  the  two  machines  have 
equal  capacities  for  electricity.  That  is, 
whether  under  similar  circumstances  they 
will  each  hold  an  equal  quantity  of  electri- 
city. 

To  determine  this,  Faraday  first  charged 
one  apparatus  only,  and  measured  the 
charge.  He  then  connected  the  two 
machines  together,  so  that  the  charge 
divided  itself  between  them.  He  then 
separated  them  and  re-measured  the  charge 
of  the  first  one.  If  the  second  apparatus 
had  the  same  capacity  as  the  first,  it  would 
have  taken  away  exactly  half  the  charge. 


ON   ELECTROSTATIC   INDUCTION.  57 

If  it  had  a  greater  or  less  capacity,  it 
would  have  taken  more  or  less  than  half 
the  charge. 

The  machines  were,  therefore,  adjusted 
till  the  charge  left  in  each  after  division 
with  the  second  was  exactly  one-half  of 
the  original  charge  before  division.  I 
have  said  that  "  the  amount  of  charge  was 
measured,"  but  have  not  yet  explained 
how  that  was  done.  Here  again  as  the 
electroscopes  which  we  have  been  experi- 
menting with  only  show  the  existence 
or  non-existence  of  a  charge,  but  do 
not  measure  its  amount,  we  require  an 
electrometer.  The  electrometer  used  by 
Faraday  to  measure  the  induction  was 
the  invention  of  Coulomb,  and  is  called 
the  "  torsion  balance."  Descriptions  of 
it  will  be  found  in  all  books  on  physics. 
The  numbers  which  Faraday  gives  as 
the  relative  strengths  of  electrifications 
are  the  number  of  degrees  of  twist  which. 


58  FOUR   LECTURES 

for  these  electrifications,  lie  read  off  on  the 
torsion  balance. 

Having  adjusted  his  torsion  balance  and 
determined  the  equality  of  his  two  Leyden 
jars,  Faraday  was  ready  to  commence  his 
measurements  of  specific  inductive  capacity. 
He  kept  one  apparatus  full  of  air,  and 
placed  in  the  other  this  hemispherical  cup 
of  shellac.  He  then  compared  the  inductive 
actions  through  the  two  machines,  and  he 
at  once  found  that  the  induction  through 
the  shellac  apparatus  was  greater  than 
that  through  the  air  apparatus  in  the  pro- 
portion of  176  to  113,  or  1-55  to  1.  In 
this  case  the  air  apparatus  had  been  charged 
first. 

Another  set  of  experiments  in  which 
the  lac  apparatus  was  charged  first  gave  a 
ratio  of  1*37  to  1.  This  difference,  which 
is  considerable,  is  accounted  for  by  the  fact 
that  the  experiment  takes  some  time,  and 
that  there  is  a  constant  leakage  of  elec- 


ON   ELECTROSTATIC   INDUCTION.  59 

tricity  going  on ;  and  in  the  one  set  the 
effect  of  the  leakage  would  be  to  give  too 
high  a  result,  and  in  the  other  to  give 
too  low  a  one.  The  mean,  therefore,  will 
not  be  far  from  the  truth. 

Faraday  gives  as  his  result  that  the  in- 
duction through  the  apparatus  half- filled 
with  shellac  is  1*5  times  that  through  the 
one  full  of  air.  From  this  he  calculates 
that  the  ratio  of  the  specific  inductive 
capacity  of  shellac  is  to  that  of  air  rather 
more  than  2  to  1. 

I  have  purposely  avoided  attempting  to 
give  the  exact  details  of  Faraday's  method 
of  working,  for  two  reasons.  One  is  that 
it  is  an  exceedingly  difficult  thing  to  under- 
stand, as  the  inductive  actions  through  the 
different  insulators  are  compared  by  an  in- 
direct method,  to  follow  which  requires  a 
considerable  familiarity  with  the  laws  of  in- 
duction ;  and  the  other  is  that  I  thought  it 
unnecessary  to  burden  your  memories  with 


60  FOUR   LECTUEES 

the  minor  details  of  a  method  of  working, 
which,  owing  to  the  invention  of  improved 
apparatus,  no  experimenter  would  now 
adopt,  wonderful  as  it  was  in  its  day,  and 
wonderful  for  all  time  as  are  the  results 
which  were  obtained  by  it. 

Faraday  continued  his  experiments  on 
other  substances,  and  here  is  a  general 
table  of  his  results  :  Shellac,  2 ;  sulphur, 
2.24;  flint  glass,  1*76  or  more. 

After  Faraday  came  numerous  experi- 
menters, who  have  published  results  more 
or  less  accordant  for  the  specific  inductive 
capacities  of  many  insulators.  As  time 
will  not  admit  of  my  giving  you  an  ac- 
count of  all  the  methods  which  have 
been  used,  and  as  for  three  years  I  myself 
have  been  engaged  in  a  determination  of 
the  specific  inductive  capacities  of  various 
substances,  I  have  preferred  to  give  you 
an  account  of  my  own  experiments  only; 
partly  because  I  believe  they  are  the  latest 


ON   ELECTEOSTATIC   INDUCTION.  61 

which  have  been  made,  but  more  par- 
ticularly as  I  shall  be  able  to  make  a  more 
interesting  lecture  about  methods  with 
which  I  am  practically  familiar,  and  which 
have,  so  to  speak,  grown  up  under  my  hand, 
than  I  could  if  I  told  you  of  methods  which 
I  have  only  read  about. 

First,  however,  let  me  tell  you  why  it  is 
so  important  that  we  should  have  accurate 
knowledge  of  the  specific  inductive  capaci- 
ties of  the  various  substances  whose  names 
you  see  in  the  table  (page  90),  india- 
rubber,  vulcanite,  paraffin -wax,  glass, 
gutta-percha,  &c. 

One  reason  is  that  these  substances 
are  much  used  for  the  insulating  parts  of 
electrical  and  telegraphic  instruments,  and 
unless  we  know  their  specific  inductive 
capacity  we  cannot  tell  beforehand  what 
their  effect  will  be  in  any  particular  in- 
strument. 

Also,  some  of  these  substances  are  used 


62  FOUR    LECTUKES 

for  the  insulators  of  submarine  cables. 
Now,  the  speed  of  signalling,  and  with  it, 
of  course,  the  gross  receipts  of  the  tele- 
graph company,  depends,  among  other 
things,  on  the  specific  inductive  capacity  of 
the  insulator,  so  it  is  as  well  to  know 
this  accurately  before  manufacturing  the 
cable.  The  speed  of  signalling  depends 
perhaps  more  upon  the  specific  inductive 
capacity  of  the  insulator  than  on  anything 
else.  The  lower  the  specific  inductive 
capacity  the  greater  the  speed.  The  great 
object  of  telegraph  engineers  at  present  is 
to  discover  a  good  insulator  of  very  low 
specific  inductive  capacity. 

But  there  is  another  and  to  us  a  far  more 
important  reason  for  desiring  accurate 
knowledge  on  this  point. 

Every  new  investigation  which  is  made 
points  to  a  close  connection  between 
electricity  and  light.  The  theory  of  their 
connection,  which  I  shall  hope  to  explain 


ON    ELECTROSTATIC   INDUCTION.  63 

in  the  next  lecture,  requires  a  certain 
relation  between  the  specific  inductive 
capacities  and  certain  optical  properties  of 
transparent  bodies.  This  theory,  which 
may  even  tell  us  what  electricity  is,  can 
only  be  tested  by  an  accurate  knowledge 
of  the  specific  inductive  capacities  of  trans- 
parent bodies. 

The  experiments  which  I  am  going  to 
tell  you  about  have  been  carried  on  under 
Professor  Clerk  Maxwell's  advice  and  su- 
perintendence, and  he  is  the  inventor  of 
the  new  method  which  has  been  used. 

The  great  difficulty  which  all  previous  ex- 
perimenters have  met  with  was  to  make  the 
experiments  quickly  enough.  If  the  elec- 
trification is  allowed  to  continue  for  even 
1 -100th  of  a  second  all  sorts  of  effects 
come  in  which  are  not  due  only  to  the 
specific  inductive  capacity  of  the  insulator. 

Three  required  conditions  were  before  us 
when  the  new  apparatus  was  being  designed. 


64  FOUR   LECTURES 

1st.  The  experiments  must  be  made 
very  quickly.  This  was  accomplished  by 
reversing  the  electrification  12,000  times 
per  second,  so  that  the  duration  of  each 
charging  of  the  insulator  was  only 
1-1 2000th  of  a  second. 

2nd.  The  electrified  metal  plates  must 
not  touch  the  insulators.  When  they  are 
allowed  to  do  so  it  is  found  that  there  is  a 
leakage  through  the  insulator  which  affects 
the  results. 

3rd.  The  method  must  be  a  zero  method. 
That  is,  instead  of  measuring  the  strength 
of  two  inductions  separately,  and  then  com- 
paring them  with  each  other,  we  must 
balance  the  two  inductions  against  each 
other  until  the  result  is  zero. 

As  an  illustration  of  the  two  methods  of 
working,  let  us  set  to  work  to  determine 
the  weight  of  a  piece  of  butter.  We  ordi- 
narily use  a  zero  method ;  that  is,  we  put 
the  butter  in  one  pan  of  a  scale,  and  vary 


ON   ELECTEOSTATIC   INDUCTION.  65 

the  weights  in  the  other,  till  there  is  no 
deflection  of  the  beam.  A  non-zero  method, 
such  as  were  the  early  determinations  of 
specific  inductive  capacity,  would  have  been 
to  have  put  the  butter  in  one  pan,  a  fixed 
weight  in  the  other,  and  to  have  endea- 
voured to  calculate  their  ratio  from  the 
deflection  of  the  beam. 

In  the  experiments  which  I  am  about 
to  describe,  the  induction  through  a  given 
thickness  of  the  substance  under  examina- 
tion is  opposed  to  the  induction  through  a 
thickness  of  air  which  can  be  varied  till 
the  two  actions  exactly  balance.  A  com- 
parison of  the  thickness  of  the  air  and  the 
substance  gives  the  specific  inductive 
capacity  of  the  latter.  The  same  electri- 
fication being  used  for  the  two  actions 
accidental  variations  in  it  do  not  affect  the 
result. 

As  the  apparatus  before  you  is  exceed- 
ingly complicated,  we  shall  do  better  to 


66 


FOUR   LECTURES 


first  study  this  diagram  (Fig.  22),  where  only 
the  most  essential  parts  are  shown.  We  may 
here  mention  that  substances  across  which 
induction  takes  place  are  called  "  dielec- 
trics," from  the  Greek  preposition 
across. 


ELECTROMETER 
FIG.E2 


DIELECTRIC 


The  induction  balance  consists  of  five 
circular  metal  plates,  a  b  c  d  e,  seen  edge- 
ways in  Fig.  22,  fixed  and  insulated  parallel 
to  each  other,  b  c  d  e  are  about  an  inch 
apart.  The  distance  from  a  to  b  can  be 
varied,  by  means  of  a  screw,  from  about 
two  and  a  half  inches  to  nothing,  ace 
are  six  inches  diameter,  b  d  four  inches. 


ON   ELECTROSTATIC   INDUCTION.  67 

a  and  e  are  connected  to  one  pole  of  the 
source  of  electricity,  c  to  the  other,  b  and 
d  are  connected  to  the  quadrants  of  a 
Thomson  (Elliott  pattern)  electrometer.* 

At  the  top  of  the  diagram  are  shown  the 
two  poles  of  the  source  of  electricity, 
which  are  always  oppositely  electrified, 
but  are  constantly  being  reversed.  The 
dielectric  under  examination  can  be  in- 
serted or  removed  at  pleasure  between  a 
and  b.  The  centre  plate  c  is  also  connected 
to  the  needle  of  the  electrometer. 

Let  us  now  suppose  that  the  dielectric 
is  removed,  and  that  all  the  plates  are 
arranged  symmetrically,  viz.,  that  distance 
c  b  equals  distance  c  d,  and  distance  e  d 
equals  distance  a  b.  Suppose  for  a  moment 
the  reversing  apparatus  stopped,  and  the 
electrification  to  be  that  of  the  upper  signs 
in  the  diagram.  Let  us  examine  the  effect 
on  the  electrometer. 

*  See  pp.  35,  36,  and  foot-note. 
F  2 


68  FOUR   LECTURES 

The  inductive  actions  of  c,  on  l>  and  d,  are 
equal  and  similar,  consequently,  the  effect 
of  c  on  the  electrometer  is  zero,  for  all 
four  quadrants  are  equally  and  similarly 
electrified  by  it.  The  inductive  action  of  a 
on  b  is  equal  and  similar  that  of  e  on  d,  and 
consequently,  the  effect  of  a  and  e  on  the 
electrometer  is  also  zero,  and  thus,  how- 
ever strongly  the  plates  are  electrified, 
there  will  be  no  deflection  of  the  electro- 
meter as  long  as  the  arrangement  is  sym- 
metrical. 

Now,  however,  let  the  dielectric  be  intro- 
duced between  a  and  b.  By  reason  of  its 
specific  inductive  capacity  being  greater 
than  that  of  air,  the  action  of  a  on  5,  which 
passes  through  it,  will  be  greater  than  that 
of  e  on  d,  and  consequently,  though  all 
four  quadrants  are  still  similarly  electrified, 
the  electrification  of  the  shaded  quadrants 
will  be  strongest,  and  the  needle  will  be 
deflected. 


ON   ELECTROSTATIC   INDUCTION.  69 

Let,  now,  the  screw  be  worked,  and 
plate  a  moved  so  as  to  increase  the  dis- 
tance a  b.  The  action  of  a  on  &  will  be 
diminished,  and  when  a  has  been  moved 
so  far  that  the  needle  has  again  come  to 
zero,  we  shall  know  that  the  increase  of 
the  distance  between  a  and  b  which  has 
been  made  by  moving  a  has  diminished  the 
induction  by  an  amount  exactly  equal  to 
the  amount  by  which  it  was  increased  by 
the  greater  action  through  the  dielectric 
under  examination. 

Knowing  the  thickness  of  the  dielectric, 
and  the  amount  which  a  has  had  to  be 
moved,  we  can  calculate  its  specific  in- 
ductive capacity.* 

*  The  formula  of  calculation  used  is  as  follows  : 
Let  us  write  K  specific  inductive  capacity,  b  thickness 
of  dielectric,  a^  reading  of  plate  a  with  only  air  in  the 
balance,  «2  reading  when  dielectric  is  inserted.  The 
action  across  a  dielectric  of  thickness  b  and  specific 
inductive  capacity  K  is  the  same  as  that  across  a  thick- 
ness of  air  — .  Hence,  when  we  introduce  the  dielectric, 


70  FOUR   LECTURES 

Let  us  now  set  the  reversing  apparatus 
to  work,  and  suppose  the  equilibrium  not 
to  be  established.  Suppose  that  we  have 
inserted  our  dielectric  but  have  not  moved  a. 

At  first  let  the  direction  of  the  electrifi- 
cation be  that  of  the  upper  signs  (Fig. 
22).  Then  there  will  be  a  deflection 
of  the  electrometer  needle  in  the  direction 

we  do  the  same  as  if  we  had  put  in  a  stratum  of  air 

thickness  — ,  and,  at  the  same  time,  take  away   the 
K. 

stratum  of  air  thickness  b,  which  is  displaced  by  it. 
Hence,  the  insertion  of  the  dielectric  is  the  same  as 
if  we  had  decreased  the  distance  between  a  and  b 

by  a  quantity  equal  to  b  —  -— .     But  to  produce  an 

lv 

equal  or  opposite  electrical  effect — i.e.  to  bring  the 
needle  to  zero — we  increased  the  distance  by  an  amount 
a2  —  0J.  Hence  these  two  quantities  are  numerically 

equal,   and   we   have   a2  —  ax  =  b  —  — ;    that    is, 

K. 

K  1  b 

. — —  j  or  K  =  . 

b       b  —  (a2  —  «i)  b  —  (az  —  aj 


ON   ELEOTEOSTATIC   INDUCTION.  71 

opposite  to  the  motion  of  the  hands  of  a 
watch,  that  is,  to  the  left.  Now  let  the  elec- 
trification be  reversed.  If  the  needle  were 
charged  in  the  ordinary  way,  and  remained 
positive,  there  would  be  a  deflection  to  the 
right.  When  the  reversals  were  rapid  the 
alternate  impulses  to  right  and  left  would 
neutralize  each  other,  and  there  would  be 
no  deflection,  however  much  the  equili- 
brium was  disturbed. 

To  get  out  of  this  difficulty  a  plan  was 
designed  which,  as  it  is  due  to  Professor 
Maxwell  and  not  to  myself,  I  may  call 
beautifully  ingenious.  Instead  of  keeping 
the  needle  permanently  charged  it  is  con- 
nected to  the  centre  plate  c  and  reverses 
with  it. 

First,  let  us  suppose  the  electrifications 
have  the  upper  signs,  and  that  by  intro- 
ducing the  dielectric  we  have  made  the 
shaded  quadrants  the  strongest.  The  force 
will  be  attractive,  and  the  needle  will 


72  FOUR   LECTURES 

turn  to  the  left.  On  reversing  the  electri- 
fications so  that  they  are  all  those  of  the 
lower  signs,  the  shaded  quadrants  will 
still  be  the  strongest,  the  force  on  the 
needle  will  still  be  attractive,  and  it  will 
still  turn  to  the  left. 

In  practical  work,  when  the  electrifica- 
tions of  the  five  plates,  the  dielectric,  the 
four  quadrants,  and  the  needle,  are  all 
being  reversed  12,000  times  per  second, 
the  needle  is  perfectly  steady,  and  so 
exactly  under  the  control  of  the  screw 
of  a,  that  a  motion  of  a  of  1-1 000th 
of  an  inch  nearer  to  or  further  from  b 
produces  a  perfectly  visible  motion  of  the 
light-spot  which  indicates  the  motions  of 
the  needle. 

We  now  turn  from  the  mathematical 
diagram  to  the  actual  instrument.  Here 
you  see  we  are  hampered  by  many  trouble- 
some conditions ;  we  have  not  only  to  say, 
"  Let  abode  be  plates  in  such  a  posi- 


ON   ELECTEOSTATIC    INDUCTION.  73 

tion,"  but  to  support  them  in  that  position 
without  the  supports  interfering  with  their 
action ;  not  only  to  say,  "  Let  them  be 
insulated,"  but  to  make  sure  that  no 
electricity  escapes ;  and  again  we  have  not 
only  to  say,  "  Let  the  dielectric  be  re- 
moved and  replaced,"  but  to  provide  means 
for  removing  it.  You  see  we  must  not 
put  our  fingers  in  between  the  plates  when 
the  apparatus  is  in  action. 

"Well,  here  is  the  apparatus  (Fig.  23),  as 
it  grew  up  in  the  course  of  three  or  four 
months'  labour.  Four  of  the  plates, 
viz.,  b  c  d  e,  are  supported  from  brass 
stages,  carried  on  brass  pillars.  Each 
plate  hangs  by  a  thin  steel  rod,  which  is 
rigid  enough  to  move  the  plate  a  little  out 
of  the  vertical,  if  required  for  adjustment. 

The  upper  end  of  each  steel  rod  passes 
through  a  hole  in  the  stage,  and  is  attached 
to  the  centre  of  a  little  ebonite  triangle  e, 
which  rests  on  the  stage  by  three  levelling 


74 


FOUR   LECTURES 


ON   ELECTROSTATIC   INDUCTION.  75 

screws  at  its  angles.  By  means  of  these 
screws,  and  ~bj  turning  the  steel  rods,  the 
plates  can  be  adjusted  so  as  to  be  exactly- 
parallel  with  each  other.  When  the  plates 
are  adjusted,  they  are  clamped  by  screws, 
which  come  down  upon  each  triangle  from 
the  stage  above.  Wires  connected  to  the 
steel  rods  lead  from  each  triangle  to  the 
binding  screws  ff,  by  which  wires  from 
other  instruments  can  be  attached  to 
them. 

The  remaining  plate  a  is  supported 
differently.  An  ebonite  block  is  fixed  at 
the  back  of  it,  and  this  is  fixed  to  a  brass 
rod  b,  about  an  inch  thick,  of  the  shape 
^} ,  and  about  six  inches  long.  This  slides 
in  two  V-shaped  grooves  in  two  brass 
pillars  b,  and  is  pressed  down  into  them  by 
springs.  It  is  moved  by  a  very  delicate 
screw,  with  a  specially  contrived  spring 
collar,  to  avoid  "  back-lash,"  as  it  is  called ; 
that  is,  to  insure  that  the  motion  of  the 


76  FOUE   LECTURES 

plate  reverses  at  the  same  instant  as  the 
motion  of  the  screw-head  is  reversed. 

A  scale  is  engraved  on  the  sliding  rod,  and 
a  vernier  on  one  of  the  supports.  This  scale 
is  read  to  l-1000th  inch  by  a  telescope 
fixed  on  the  case  of  the  instrument  behind 
g  (it  cannot  be  seen  in  the  engraving). 
The  scale  is  illuminated  by  a  candle. 

The  dielectrics  k  k  are  made  seven  inches 
square,  and  their  thickness  varies  from 
one-quarter  inch  to  one  inch. 

The  slide  for  inserting  and  withdrawing 
the  dielectrics  is  shown,  c  d,  in  the  picture. 
It  moves  in  guides,  and  can  be  pulled  in 
and  out  by  the  square  handle  without 
opening  the  case,  or  disturbing  the  ex- 
periment in  any  way.  The  three  other 
handles  are  for  adjusting  the  position  of 
the  dielectric ;  one  moves  it  parallel  to  itself 
by  means  of  a  rack,  one  turns  it  round  a 
vertical  axis  by  a  tangent  screw,  and  the 
third  round  a  vertical  axis  by  screwing 


ON   ELECTROSTATIC   INDUCTION.  77 

in    or   withdrawing   a   wedge   under   one 
side. 

The  callipers  m  shown  in  the  same  figure 
are  for   measuring    the    thickness   of  the 
dielectrics,   and   for   adjusting   the   plates     f- 
parallel   to    each   other.      For   this   latter     f. 
purpose  they  are  laid  on  the  hinged  bracket, 
n,  which  can  be  fixed  to  a  socket  at  the 
back  of  the  apparatus. 

In  work  all  the  upper  stage  which  con-     .*- 
tains   the   connections   of  the    plates   re-     £' 
ceiving   the   induction   is    enclosed    in    a     ^ 
metal  box  connected   to  earth,  the  wires 
leading  to  the    electrometer  are  enclosed 
in  a  metal  tube,  and  the  electrometer  itself 
is  in  a  metal  case.     The  reason  of  this  is 
to   protect  these  parts   of   the  apparatus 
from  accidental   induction   from  the  con- 
necting wires,  &c.     If  from  any  accidental 
cause  the  earth  connection  is  interrupted, 
the  effect  is  at  once  seen  in  the  uncertain 
and  irregular  behaviour  of  the  instrument. 


78  FOUR   LECTURES 

The  metal  cover,  h,  first  used  for  the  upper 
stage,  is  shown.  It  was  made  of  card  and 
tinfoil.  A  brass  one  is,  however,  now  used 
instead  of  it. 

Before  going  any  further  I  must  express 
my  obligations  to  Mr.  Kieser,  of  the  firm 
of  Elliott  Brothers,  for  the  admirable  way 
in  which  he  has  constructed  the  instrument 
from  my  drawings. 

The  electrometer  is  of  the  ordinary 
Elliott  pattern  quadrant  described  by  me  in 
the  last  lecture  in  the  account  of  Hopkin- 
son's  experiment. 

The  source  of  electricity  is  a  large 
induction  coil  by  Apps,  having  twenty- 
two  miles  of  secondary  wire  (Fig.  24), 
which,  with  a  suitable  battery  and  break, 
is  capable  of  giving  a  seventeenth-inch 
spark  in  the  air.  In  these  experiments  it 
is,  however,  used  in  a  different  manner. 
The  object  is  to  obtain  a  moderate  electri- 
fication, very  rapidly  reversed,  and  to 


ON   ELECTROSTATIC   INDUCTION. 


79 


insure  that  the  positive  and  negative  elec- 
trifications shall  have  equal  strengths. 

When  a  coil  is  worked  in  the  ordinary 
way,  although  the  same  quantities  of  elec- 


FIG.   24. 


tricity  are  produced  both  on  making  and 
breaking  the  primary,  yet  the  arrangement 
of  the  currents  is  such  that  the  current 
produced  in  one  direction  on  breaking  will 
produce  a  much  stronger  external  effect 


80 


FOUR   LECTURES 


than  that  produced  in  the  other  direction 
on  making.  To  obtain  equal  electrifications 
in  the  two  directions  it  is  necessary  to  use 
a  very  large  coil,  a  very  small  battery,  and 
a  very  rapid  break. 


APPROXIMATE  SCALE  OF  INCHES, 
FIG.   25. 

In  these  experiments  the  current  in  the 
coil  primary  is  only  that  of  ten  small 
Leclanche  cells.  The  rapid  break  is  shown 
in  Fig.  24,  and  also  on  a  larger  scale  in 
Fig.  25.  It  consists  of  a  little  electro-mag- 


ON   ELECTROSTATIC   INDUCTION.  81 

netic  engine.  The  scale  of  inches  in  Fig.  25 
shows  the  size.  One  electro-magnet  is 
fixed,  and  another  revolves.  A  commutator 
is  so  arranged  that  the  force  between 
approaching  poles  is  always  attractive,  and 
that  between  poles  which  are  moving  apart 
repulsive.  When  the  engine  is  worked  by 
four  quart-sized  Grove  cells  the  flywheel 
revolves  just  100  times  per  second.  You 
see  the  whole  engine  is  not  more  than 
eight  inches  long  and  four  high  and  broad, 
yet  when  I  set  it  in  motion  the  hum  and 
vibration  are  felt  all  over  the  building. 

In  the  rim  of  the  flywheel,  which  is  about 
two  inches  diameter,  are  60  slits  cut,  into 
each  of  which  is  let  a  piece  of  ebonite.  A 
light  spring  presses  on  it,  and  the  primary 
current  on  its  way  from  the  Leclanche 
battery  to  the  coil  has  to  pass  from  the 
spring  to  the  wheel.  It  is  thus  broken 
sixty  times,  and  closed  sixty  times  in  each 
revolution  of  the  wheel.  At  each  "break" 

G 


82  FOUR   LECTURES 

a  current  is  induced  in  one  direction  in  the 
secondary,  and  at  each  "make"  one  is  induced 
in  the  other  direction.  Thus  there  are  120 
alternating  currents  induced  every  revolu- 
tion of  the  engine,  and,  as  the  engine  turns 
100  times  per  second,  there  are  12,000 
currents  each  second.  The  engine  does 
great  credit  to  its  maker,  Mr.  Apps. 

To  test  the  equality  of  the  currents  in 
the  two  directions,  the  secondary  poles 
were  connected  to  a  small  vacuum  tube. 
No  effect  whatever  was  produced  on  the 
light  by  reversing  the  primary  by  means  of 
a  commutator  between  the  engine  and  the 
Leclanche  battery. 

One  of  the  secondary  poles  of  the  coil  is 
connected  to  plate  c  of  the  induction  balance 
and  the  other  to  the  plates  a  and  e,  and  of 
course  the  electrifications  of  every  part  of 
the  balance  and  electrometer  are  reversed 
with  those  of  the  coil  poles.  The  strength 
of  the  electrification  is  such  that  a  spark 


ON    ELECTROSTATIC   INDUCTION. 


83 


could  be  obtained  between  the  coil  poles  of 
from  l-25th  to  l-50th  of  an  inch;  that  is, 
it  is  about  equal  to  the  direct  discharge  of 
2000  silver  cells. 

In  case,  however,  the  sum  of  the  make 


APPROXIMATE  SCALE 
OF  INCHES 


FIG.  26. 


and  break  electricities  should  not  be  exactly 
equal,  the  machine  (Fig.  26)  was  introduced 
for  reversing  the  secondary  current  about 
thirty  times  per  second  on  its  way  from  the 
coil  to  the  induction  balance. 

It  consists  of  a  wire  frame,  which  dips 
G  2 


84 


FOUR    LECTURES 


RECESS  IM  WALL  CARRYING  WIRES 

I  B*TTEBItS          I]       l|||  I     UPSTMPS  WAUL 


SCALE  OF  FEET 


FIG.  27- 


ON   ELECTROSTATIC   INDUCTION.  85 

alternately  into  different  mercury  cups,  and 
so  reverses  the  current.  It  is  worked  from 
a  crank  on  the  axis  of  a  small  engine, 
similar  to  that  of  Fig.  25.  The  speed  is 
regulated  by  a  friction  brake,  consisting 
of  a  silk  loop  round  a  pulley  on  the  axle, 
to  which  an  india-rubber  band  is  attached ; 
a  cord  from  this  band  is  wound  round  a 
bradawl,  driven  into  the  base ;  by  turning 
the  bradawl,  the  speed  can  be  exactly 
regulated.  This  engine  is,  however,  not 
now  used,  as  the  reversals  given  by  the 
rapid  break  are,  I  believe,  perfect. 

This  plan  (Fig.  27)  shows  the  arrangement 
of  the  apparatus  in  my  laboratory.  You 
see  that,  standing  where  I  can  see  the 
scale,  I  have  the  key  of  the  coil  primary 
under  my  right  hand,  and  the  mechanical 
slide  and  the  screw  of  a,  under  my  left. 
Compare  this  diagram  with  Fig.  22.' 

Here  in  the  lecture-room  the  apparatus 
is  arranged  in  nearly  the  same  way,  except 


86  FOUR   LECTURES 

that,  instead  of  the  small  scale  and  paraffin 
lamp,  we  have  a  large  scale  and  lime-light, 
so  that  you  may  see  the  deflections  of  the 
electrometer,  and  that  Mr.  Cottrell  has 
hinged  a  long  wooden  pointer  to  the  induc- 
tion balance,  so  that  you  can  all  see  the 
motions  of  plate  a. 

We  will  now  make  an  actual  determina- 
tion of  the  specific  inductive  capacity  of  this 
beautiful  slab  of  "  double  extra  dense  flint  " 
glass.  I  place  it  on  the  slide  and  draw 
out  the  latter,  so  that  the  glass  is  not 
between  the  plates,  and  now  put  plate  a 
into  what  I  think  will  be  about  the  position 
of  equilibrium.  We  start  the  engine  and 
make  contact  in  the  primary  circuit,  and 
you  see  there  is  a  small  deflection.  I  move 
a  backward  and  forward  till  the  light-spot 
comes  to  zero  and  remains  there  as  we  make 
and  break  the  primary.  We  now  read  the 
scale  of  a  and  write  down  the  result  as  ax. 

Now  I  push  the  handle  of  the  slide  and 


ON   ELECTROSTATIC   INDUCTION.  87 

insert  the  dielectric  between  a  and  b.  You 
see  that  there  is  at  once  a  large  deflection 
to  the  left.  I  work  the  screw  of  a,  and 
draw  it  further  away  from  &,  and  here  you 
see  is  the  light-spot  coming  back  to  zero, 
and  now,  when  I  have  moved  a  about  an 
inch,  I  have  adjusted  the  electrometer 
exactly  to  zero.  We  take  the  reading 
again  and  write  it  down  as  &2.*  The  dif- 
ference between  the  two  readings  is  the 
motion  of  a,  and  knowing  this  and  the 
thickness  of  the  glass,  we  can  calculate  its 
specific  inductive  capacity  by  the  formula 
given  in  the  foot-note  to  page  69. 

*  In  practice  the  probable  position  of  a2  is  always 
obtained  by  a  preliminary  experiment,  and  the  plate  a 
put  there  before  making  contact  with  the  dielectric 
inserted,  so  that  there  is  never  any  large  deflection.  It 
is  found  that  when  large  deflections  take  place,  the 
needle  does  not  always  return  to  quite  the  same  zero, 
In  the  experiment  shown  to  the  audience,  however, 
this  was  not  done  in  order  that  the  disturbance  of 
equilibrium  caused  by  inserting  the  dielectric  might  be 
more  clearly  seen. 


00  FOUR  LECTURES 

On  drawing  out  the  glass,  there  is,  of 
course,  a  deflection  to  the  right,  but  you 
notice  it  happens,  from  some  defect  in  the 
adjustment  of  the  electrometer,  that  this 
deflection  is  very  much  smaller  than  was 
the  previous  deflection  to  the  left. 

Here,  however,  we  may  see  the  beauty  of 
the  zero  method,  for  we  have  nothing  to  do 
with  the  magnitude  of  the  deflection,  and 
we  see  that  if  we  bring  the  needle  back  to 
zero  by  screwing  in  a,  we  require  exactly 
as  much  inward  motion  to  compensate  this 
small  deflection  to  the  right  as  we  previ- 
ously required  outward  motion  to  compen- 
sate the  large  deflection  to  the  left.  In 
fact,  when  the  light-spot  is  again  at  zero, 
the  reading  of  a  is  precisely  the  same  as  it 
was  at  the  beginning  of  the  experiment.  • 

In  actual  work,  successive  readings 
never  disagree  by  more  than  two,  or  at 
most  three  thousandths  of  an  inch.  We 
cannot,  however,  get  the  same  accuracy 


ON   ELECTROSTATIC    INDUCTION.  89 

here,  as  the  rapid  break,  and  the  audience 
are  all  supported  on  the  same  floor  as  the 
induction  balance  and  electrometer,  and  so 
cause  vibrations.  In  my  own  laboratory 
the  induction  balance  and  electrometer  are 
supported  on  brick  and  slate  piers  quite 
independent  of  the  floor  on  which  the  break- 
engine  and  the  observer  stand.  Here  on 
this  diagram  (page  90)  are  the  results  of 
my  experiments,  extracted  from  my  paper 
which  has  recently  been  read  before  the 
Royal  Society.* 

If  we  look  at  the  experiments  on  ebonite, 
we  shall  see  that  we  get  substantially  the 
same  results  with  dielectric  plates  of  very 
different  thicknesses.  This  is  a  very  good 
test  of  the  accuracy  both  of  the  instrument 
and  the  formula  of  calculation.  The  ex- 
periments on  paraffin  wax  agree  very  well 
with  each  other.  The  most  accurate  deter- 
minations, which  have  been  recently  made 
*  Proc.  Boy.  Soc.,  191,  1878,  p.  155. 


90 


FOTJB   LECTURES 


Dielectric. 

Glass,  Slabs  about  C  Double  extra  dense  flint 

1  inch  thick.       \  Extra  dense  flint     .     . 
Chance's     optical  1  Light  flint     .... 
I  Hard  crown  . 


Specific  Inductive 
Capacity. 

3-1639 
3*0536 
3-0129 
3-1079 


Common     plate, 
two  slabs. 


No.  1 
No.  2 


!No.  1 
No.  2 
No.  3 
No.  4 

Best  quality  gutta-percha 
Chatterton's  compound    . 


3-2581 
3-2282 

2-2697 
2-2482 
2-3097 
2-3077 


3-2431 


.  2-2838 


2 -46  2 5 
2-5474 


Mean     1-9936 


Solid  paraffin,  sp.  / 
gr.atll°C.-9109.  ' 
Melting       point 
68°  C.    Six  slabs 
cut    in    planing 
machine.         Ee- 
sults      corrected 
for  cavities. 

Shellac 2'7464 

Sulphur 2-5793 

Bisulphide  of  carbon *  1-8096 


No.  1 

No.  2 
No.  3 

No.  4 
No.  5 
No.  6 


1-9940 
1-9784 
1-9969 
2-0126 
1-9654 
2-0143 


*  I  am  not  quite  certain  of  the  accuracy  of  this 
result. 


ON   ELECTROSTATIC   INDUCTION.  91 

of  specific  inductive  capacity,  have  been 
those  of  Messrs.  Gibson  and  Barclay,*  who 
experimented  on  paraffin  only.  They  used 
a  method  entirely  different  from  mine,  and 
found  that  the  specific  inductive  capacity 
of  their  paraffin  was  T977.  Correcting 
for  a  slight  difference  of  density,  I  find  that 
if  they  had  used  my  paraffin  their  result 
would  have  been  1*9833,  which  differs  from 
my  result  by  only  one-half  per  cent.,  or  one 
part  in  two  hundred. 

*  Phil.  Trans.,  1871,  p.  573. 


LECTURE    IV. 

FEB.  6. 

SPECIFIC  INDUCTIVE  CAPACITY  OF  GASES. 
ELECTEO-MAGNETIC  THEOEY  OF  LIGHT. 

FAEADAY*  made  a  great  number  of 
experiments  on  the  specific  inductive 
capacities  of  gases,  to  see  if  lie  could  detect 
any  differences  between  them,  but  he  was 
unable  to  do  so.  He  compared  no  less 
than  twenty-five  pairs  of  gases.  He  also 
compared  dry  and  damp  air,  and  hot  and 
cold  air,  and  air  at  various  pressures,  but 
with  his  apparatus  he  could  detect  no 
difference  at  all. 

It  is  curious  to  see  how  long  and  how 
earnestly  he  sought  for  evidence  of 
differences  of  action. 

*  "  Experimental  Researches,"  vol.  i.  p.  406. 


ON   ELECTROSTATIC   INDUCTION.  93 

He  seemed  to  have  so  strong  an  instinct 
that  there  ought  to  be  a  difference,  that 
he  literally  struggled  against  the  evidence 
that  every  experiment  he  made  seemed  to 
pile  up  against  his  theory.  He  knew  that 
gases  differed  in  so  many  of  their  other 
physical  properties  that  he  could  hardly 
believe  they  were  all  alike  in  this  one.  He 
used  the  same  apparatus  as  I  showed  you 
last  lecture.  It  was  not  till  1877  that 
Professors  Ayrton  and  Perry,*  working 
with  apparatus  many  thousand  times  more 
delicate  than  that  which  was  at  Faraday's 
disposal,  succeeded  in  showing  that  the 
reason  why  Faraday  had  not  been  able  to 
detect  differences  of  specific  inductive 
capacities  in  gases  was  not  that  these 
differences  did  not  exist,  but  that  they 

*  "  On  the  Specific  Inductive  Capacity  of  Gases." 
Paper  read  before  the  Asiatic  Society  of  Japan,  April 
18,  1877.  Printed  at  the  Japan  Mail  Office,  Yoko- 
hama. 


94  POUR   LECTUEES 

were  too  small  to  be  detected,  except  by  a 
quadrant  electrometer.  Professors  Ayrton 
and  Perry  have  not  only  shown  that 
different  gases  have  different  specific 
inductive  capacities,  but  that  the  specific 
inductive  capacity  of  the  same  gas  is 
different  at  different  temperatures  and 


ELEVATION. 


ciJwjwo.S^^^ 

FIG.    29. 


FIG.    28. 

pressures  ;  and,  further,  they  have  actually 
measured  the  amounts  of  these  differences. 
Through  the  kindness  of  Professor  Ayrton 
I  am  able  to  explain  to  you  his  method  of 
working ;  but  I  cannot  show  you  his 
experiments,  as  they  were  made  in  Japan, 
and  the  apparatus  being  the  property  of 


ON   ELECTROSTATIC   INDUCTION.  95 

the  Japanese  Government,  had  to  be  left 
behind  when  Professor  Ayrton  returned 
last  August. 

Professor  Ayrton5 s  method  of  working 
was  as  follows : — He  prepared  two  con- 
densers— that  is,  Leyden  jars — into  one  of 
which-  he  could  put  different  gases  as  the 
insulator;  one, which  he  called  the  "  open  air 
condenser "  (Figs.  28,  29),  consisted  of  a 
thick  brass  plate,  Z,  laid  on  the  table,  Y, 
having  over  it  another  brass  plate,  W.  This 
latter  was  supported  on  three  ebonite 
levelling  screws.  The  two  brass  plates 
formed  the  conductors,  and  the  air  between 
them  formed  the  insulator.  You  will  see 
the  use  of  this  condenser  immediately. 

The  other  condenser  was  called  the  "  closed 
condenser."  (Figs.  30,  31,  32).  It  consisted 
of  eleven  brass  plates,  fixed  parallel  to  each 
other,  in  a  metal  box.  Nos.  1,  3,  5,  7,  9,  1 1 
were  all  connected  to  one  piece  of  metal, 
and  Nos.  2,  4,  6,  8,  10  to  another.  The 


96 


POUR  LECTUEES 


two  sets  of  plates  were  insulated  from  each 
other.  They  thus  formed  a  condenser  of 
very  large  surface,  and  all  the  spaces 


SIDE  ELEVATION 


FIG.    30. 


END   ELEVATION. 

FIG.    31. 


between  could  be  filled  with  the  gas  under 
examination.  The  brass  box  was  closed 
quite  air-tight,  and  could  be  connected  to 


ON    ELECTROSTATIC   INDUCTION.  97 

an  air-pump,  by  the  tube  R,  when  required. 
The  induction  through  this  apparatus, 
when  it  contained  different  gases,  could  be 
compared.  But  it  wo  aid  be  impossible  to 
preserve  a  perfect  record  of  the  action 
through  the  condenser,  and  to  insure  that 
the  inducing  force  should  always  be  the 
same.  For  this  reason  the  "  open  con- 
denser "  before  mentioned  was  used. 

The  closed  condenser  was  filled  with 
dry  air  at  ordinary  pressure,  and  the  in- 
duction through  it  was  compared  by  means 
of  a  very  delicate  quadrant  electrometer 
with  that  through  the  open  condenser. 
This  ratio  was  noted. 

.  The  closed  condenser  was  then  filled 
with  some  other  gas,  and  the  ratio  again 
noted  between  it  and  the  open  condenser. 
The  open  condenser,  of  course,  remained 
the  same.  The  ratio  of  these  two  results 
gave  the  ratio  of  the  inductive  actions 
through  air  and  the  gas  under  examination. 

H 


98  FOUR   LECTURES 

The  open  condenser,  in  fact,  only  acted 
as  a  standard  measure,  just  as  if  it  were 
desired  to  compare  the  lengths  of  two 
ropes,  and  it  was  not  convenient  to  lay 
them  together,  they  could  each  separately 
be  compared  with  a  yard  measure. 

The  difficulties  of  this  investigation 
must  have  been  enormous. 

Professor  Ayrton's  paper  contains  quite 
a  heartrending  list  of  breakages,  leakages, 
twistings.  He  relates  how,  first,  the  box 
was  not  air-tight.  Then  a  smith,  having 
been  sent  for  to  solder  it,  his  hot  tools 
damaged  the  ebonite  inside,  and  it  all  had 
to  be  taken  to  pieces  again.  Then  the  long 
glass  tube,  P  M,  in  which  the  conducting 
wire,W,was  insulated  broke  again  and  again. 
Then,  when  all  was  finished,  mercury  from 
the  air-pump  got  inside,  and  spoilt  the 
whole  affair. 

"We  in  England  suffer  in  the  same  way ; 
but  can  we  imagine  what  the  difficulties 


ON   ELECTROSTATIC   INDUCTION.  99 

of  delicate  physical  investigation  must  be 
when  all  repairs  of  instruments  have  to  be 
done  either  by  the  experimenter  himself, 
or  by  Japanese  workmen? 

However,  at  last  the  skill  and  invincible 
patience  of  the  investigators  conquered 
all  difficulties,  and  the  results  on  this 
diagram  were  obtained  : — 

AYRTON  AND  PERRY  ON  SPECIFIC  INDUCTIVE  CAPACITIES 
OF  GASES. 

Specific 

Dielectric.  Inductive 

Capacity. 

Hydrogen  .  .  .  0-9998 
Coal  Gas  .  .  .  1-0004 
Sulphuric  dioxide  1-0037 

These  results  are  a  marvel  of  experi- 
mental work. 

In  some  cases  the  whole  difference  which 
had  to  be  measured  was  only  two  parts  in 
10,000.  The  maximum  difference  was 
only  thirty-seven  in  10,000,  and  yet  these 
differences  have  not  only  been  observed, 
but  have  been  measured  so  accurately  that 
H  2 


Specific 

Dielectric.  Inductive 

Capacity. 

Air 1-0000 

Vacuum      .     .     .    0-9985 
Carbonic  dioxide     1*0008 


100  FOUR   LECTURES 

a  mathematical  comparison  of  the  various 
observations  of  which  they  are  the  means 
shows  that  the  probable  error  is  not  more 
than  five  parts  in  100,000,  or  l-200th  per 
cent. 

In  future  it  will  not  be  sufficient  to  say, 
the  specific  inductive  capacity  of  air  is 
taken  as  unity.  We  must  specify  the 
pressure  and  temperature  of  the  air,  and 
say  that  the  unit  specific  inductive  capacity 
is  that  of  dry  air  at  30  inches  barometer 
and  32°  Fahr.  temperature. 


We  now  come  to  perhaps  the  most 
important  and  interesting  part  of  our 
subject;  namely,  the  relations  between 
electricity  and  light. 

It  is  chiefly  by  the  consideration  of  the 
connections  which  may  exist  between 
different  forms  of  physical  energy  that  we 
may  hope  to  some  day  obtain  a  clearer 
notion  of  their  actual  nature. 


• 


ON   ELECTROSTATIC   INDUCTION.          101 

Between  light  and  electricity  there  are 
numerous  and  close  relations  and  analogies. 
The  form,  however,  in  which  electricity  is 
most  intimately  connected  with  light,  is 
not  the  static  form,  which  we  have  been 
considering,  but  another,  called  "  electro- 
dynamic,"  or,  because  of  its  magnetic 
properties,  "  electro  -magnetic."  Hitherto 
in  these  lectures  we  have  considered  only 
bodies  charged  with  electricity  at  rest. 
We  have  examined  the  induction  of  charged 
bodies,  that  is,  bodies  containing  electricity, 
but  containing  a  certain  quantity  not  in 
motion.  The  only  cases  of  the  motion  or 
flow  of  electricity  which  we  have  noticed 
have  been  the  momentary  motions  which 
have  accompanied  the  discharging  or 
charging  of  conductors. 

When,  however,  electricity  instead  of 
being  at  rest,  is  flowing  as  a  current — when, 
for  instance,  electricity  is  being  drawn  out 
of  one  end  of  a  wire  and  constantly  being 


'102  FOUR  LECTURES 

renewed  at  the  other — it  produces  an  en- 
tirely new  set  of  inductive  actions,  different 
from  those  which  we  have  been  considering. 
Now  I  am  not  going  to  attempt,  in  the 
half-hour  which  remains  of  the  last  of  these 
,  lectures,  to  give  an  account  of  the  laws  of 
•\:;*  electro-magnetic  induction ;  but  this  much 
-will  be  necessary,  and,  I  think,  sufficient 
as  an  introduction  to  our  study  of  the 
relations  between  electric  induction  and 
light. 

Electric  (that  is,  electro- static)  phenomena 
are  so  closely  linked  in  every  detail  with 
electro-magnetic  ones,  that  any  arguments 
which  show  that  the  same  mechanism 
transmits  electro-magnetic  induction  and 
light  will  also  hold  for  electro -static 
induction,  though  I  do  not  say  but  that 
the  working  of  the  machinery  may  be  very 
different  in  the  two  cases. 

"  One  mechanism  for  electric  induction 
and  light."  This  is  Professor  Clerk  Max- 


ON    ELECTROSTATIC   INDUCTION.  103 

well's  theory,*  which  every  new  measure- 
ment which  is  made  helps  to  render  more 
probable. 

It  is  this  theory  which,  when  it  is  finally 
proved,  and  when  some  difficulties  that  now 
beset  it  are  cleared  away,  as  no  doubt  they 
will  be,  will  tell  us  "  what  is  light,"  "  what 
is  electricity."  Let  us  consider  it,  and  try 
and  understand  what  it  means. 

I  will   give   you   the  substance   of 
preface  to  Maxwell's  chapter  on  the  subject,    Q 
and  then  explain  to  you  such  of  his  argu- 
ments as  I  understand  well  enough  to  put 
into  an  unmathematical  form.  P 

Light  is,  all    men   of    science  are  now     CO 
agreed,  a  wave  motion  of  a  medium  which    lr-4 
we  call  the  ether,  which  fills  all  space,  and 
probably  permeates  all  bodies. 

The  sun  expends  energy,  and  sends  it 
off  from  him.  This  energy  travels  through 

*  "  Electricity  and  Magnetism,"  by  J.  Clerk  Max- 
well, F.K.S.,  chapter  xx.,  vol.  ii.  p.  383. 


104  FOUR   LECTUEES 

the  dark  planetary  spaces  until  it  falls  on 
the  eye,  and  then  it  is  felt  as  light.  When 
it  falls  on  opaque  bodies,  part  is  reflected 
from  them,  and,  falling  on  the  eye,  is  felt 
as  the  light  which  makes  them  visible. 
Part  penetrates  them,  and  heats  them. 
In  what  form  did  this  energy  which  we 
know  as  light  and  radiant  heat  exist  in  the 
dark  space  between  the  sun  and  earth  ? 

The  undulatory  theory  of  light  answers 
that  this  dark  space  is  full  of  a  medium,  a 
very  thin  fluid,  and  that  the  energy  given 
off  from  the  sun  is  expended  in  producing 
waves  in  that  portion  of  the  medium  next 
it,  which  in  their  turn  expend  the  energy 
they  have  received  in  producing  waves  in 
the  next  portions;  and  so  the  energy  is 
transmitted  by  these  wave  motions,  until 
on  striking  the  earth  or  the  eye  it  becomes 
heat  or  light. 

We  have  said  that  electro-static  induction 
is  a  strain  or  distortion  of  the  insulator 


ON  ELECTROSTATIC   INDUCTION.  105 

through  which  it  is  transmitted.  This 
medium  may  be  air,  or  glass,  or  paraffin, 
&c.,  as  we  saw.  But  what  is  the  medium 
when  the  sun  acts  inductively  on  the  earth, 
as  no  doubt  he  does  ? 

When  a  sun-spot  bursts  out  into  stronger 
activity  all  the  magnetic  instruments  at 
Kew  Observatory  move  in  sympathy  with 
it. 

What  is  the  medium  which  transmits 
this  electro-magnetic  induction  from  the 
sun  to  the  earth  ?  Professor  Maxwell  says 
that  it  is  one  and  the  same  medium  as  that 
which  carries  the  waves  of  light,  or  that 
light  itself  is  an  electro-magnetic  disturbance. 

Let  us  now  consider  some  of  the  argu- 
ments which  have  led  up  to  this  theory. 

It  is  proved,  I  think,  that  electric 
induction  is  a  strain  of  some  kind ;  and, 
when  electric  induction  passes  through 
space  in  which  there  is  not  any  ordinary 
matter,  we  agree  to  call  the  unknown 


106  FOUR   LECTUEES 

something  that  fills  the  space  and  transmits 
the  strain  an  "  ether." 

Light  is  a  strain  of  some  kind;  and 
when  light  passes  through  space  where 
there  is  not  any  ordinary  matter,  we  agree 
to  call  the  unknown  something  that  fills 
the  space  and  transmits  the  strain  an 
"  ether." 

One  word  of  explanation  of  the  term 
"strain."  In  physics  this  word  has  a 
more  extended  meaning  than  in  common 
language.  Any  change  of  form  whatever 
is  called  a  strain.  A  wave  motion  would, 
therefore,  be  called  a  strain. 

How  shall  we  decide  whether  these  two 
ethers  are  one  and  the  same  ?  "We  must 
examine  and  measure  as  many  of  the 
properties  of  each  ether  as  we  can;  and 
then,  if  we  find  that  all  the  properties  are 
the  same,  we  shall  be  sure  that  the  ethers 
are  not  two  but  one. 

If,  again,  we   find   that   most  of  their 


ON   ELECTEOSTATIC   INDUCTION.          107 

properties  nearly  agree,  but  not  quite,  we 
must  reserve  our  judgment ;  but  we  might 
in  that  case  be  allowed  to  speculate  on  the 
possibility  of  the  same  ether  sea  vibrating 
somewhat  differently  when  disturbed  by 
electricity  or  by  light. 

One  important  point  of  resemblance 
appears  at  once.  In  the  case  of  light,  the 
researches  of  Young,  Fresnel,  Huygens,  and 
Green  have  shown  that  the  energy  in  the 
medium  is  partly  "  potential "  and  partly 
"  kinetic.'' 

These  are  two  hard  words,  but  I  think  I 
can  make  their  meaning  clear  to  you. 

"  Kinetic >}  energy  is  the  energy  of 
motion.  "  Potential "  energy  is  the  energy 
of  strain.  A  stone  when  falling  has  kinetic 
energy.  By  virtue  of  its  motion  it  can 
strike  a  hard  blow  on  anything  that  comes 
in  its  way  and  stops  that  motion. 

The  same  stone  suspended  by  a  string 
has  potential  energy,  because  the  instant 


108  FOUR   LECTURES 

that  the  string  is  cut  it  will  acquire  kinetic 
energy.  We  may  regard  the  earth  and 
the  stone  together  as  a  system  which  was 
strained  when  the  stone  was  pulled  away 
from  the  earth  against  the  action  of  gravi- 
tation, and  when  the  state  of  strain  is 
released,  energy  is  developed  as  the  stone 
falls. 

Another  illustration.  Suppose  a  loco- 
motive in  motion,  with  full  steam  up. 

Now,  let  the  steam  be  shut  off  and  the 
brakes  applied.  The  engine  does  not  stop 
at  once,  because  it  has  kinetic  energy,  that 
is,  energy  due  to  its  motion.  It  does  not 
stop  until  the  whole  of  this  energy  has 
been  expended  in  heating  the  brakes  and 
the  rails. 

"When  it  is  at  a  standstill,  is  all  the  energy 
expended  ?  No,  it  is  not,  for  even  without 
burning  any  more  coal,  we  have  only  to 
turn  on  the  tap,  and  the  potential  energy 
of  the  compressed  or  strained  steam  in  the 


ON   ELECTEOSTATIC   INDUCTION.          109 

boiler  is  released,  and,  as  the  engine  starts, 
is  changed  from  potential  into  kinetic 
energy,  and  the  motion  continues  until  this 
again  is  expended  in  heating  the  brakes 
and  rails. 

Well,  as  I  said,  in  the  luminiferous  ether, 
when  carrying  light  vibrations,  these  two 
sorts  of  energy  exist.  The  ether  is  in  rapid 
vibrating  motion,  so  has  kinetic  energy. 
It  is  also  in  a  state  of  strain,  so  has  poten- 
tial energy.  Note  that  I  only  tell  you 
this,  and  have  not  given  you  any  proof. 
The  proof  is  a  complex  bit  of  mathematics. 

The  electric  ether  is  also  a  receptacle  of 
the  two  forms  of  energy,  potential  and 
kinetic;  and  here  we  can  actually  partly 
separate  them  and  study  them  apart,  for 
when  static  induction  or  the  induction  of 
electricity  at  rest  is  going  on,  the  ether  is 
strained,  but  is  not  being  kept  in  motion  ; 
while  when  electro -magnetic  induction,  that 
is,  the  induction  of  flowing  currents  of 


110  FOUft   LECTURES 

electricity  is  going  on,  motions  are  pro- 
duced and  kept  up  in  the  ether,  that  is,  it 
receives  kinetic  energy. 

In  the  second  case,  however,  it  receives 
potential  energy  also. 

Possibly,  the  fact  that  in  the  electri- 
cal case  we  can  partly  separate  the  two 
forms  of  energy  may,  at  some  future  time, 
throw  light  on  the  distribution  of  the 
potential  and  kinetic  energies  in  the  optical 
case. 

There  is  another  very  important  point  of 
resemblance  between  electric  and  electro- 
magnetic induction,  on  the  one  hand,  and 
light  on  the  other. 

It  is  this,— 

In  light  it  is  known  that  the  waves  are 
at  right  angles  to  the  direction  of  the 
ray. 

I  mean  that  if  a  ray  of  sun-light  falls 
vertically  on  the  earth,  its  vibrations  are 
all  horizontal. 


ON   ELECTROSTATIC   INDUCTION.  Ill 

You  know  that  in  this  it  differs  from 
sound,  as  the  vibrations  of  the  air  which 
are  sound  are  in  the  same  line  as  that  in 
which  the  sound  is  travelling. 

It  becomes  of  great  interest  to  determine 
in  which  direction  the  electric  disturbance 
takes  place.  If  I  hold  this  rod  over  this 
electroscope  so  that  the  line  of  force  acts 
vertically  downwards,  then  are  the  vibra- 
tions of  the  ether  vertical  or  horizontal  ? 

Professor  Clerk  Maxwell  has  mathemati- 
cally investigated  this  point,  and  has  shown 
that  the  disturbances  both  of  electro-static 
and  electro -magnetic  induction  exactly 
agree  with  those  of  light  in  this  respect, 
for  they  are  both  at  right  angles  to  the 
direction  of  the  ray  of  electric  or  magnetic 
induction. 

Further,  he  has  shown  that,  if  electro- 
static and  electro-magnetic  induction  take 
place  together,  the  electric  disturbance  will 
always  be  at  right  angles  to  the  magnetic 


112 


FOUR   LECTUEES 


one ;  that  is,  if  the  direction  of  the  induc- 
tion be  vertical,  the  direction  of  the  disturb- 
ances will  be  horizontal,  and  if  the  direction 
of  one  of  these  horizontal  disturbances  (say 


FIG.    33. 


the  electric  one)  be  from  N.  to  S.,  that  of 
the  magnetic  one  will  be  from  E.  to  W. 

This  diagram  (Fig.  33)  is  from  Professor 
Maxwell's  book.* 

*  "Electricity,"  vol.  ii.  p.  390,  fig.  66. 


ON   ELECTROSTATIC   INDUCTION.  113 

Another  argument  in  favour  of  the  theory 
is  that  it  gives  a  real  mathematical  reason 
for  the  fact  that  all  good  true  conductors 
are  exceedingly  opaque.  All  metals,  for 
instance,  conduct,  and  are  opaque.  The 
conduction  of  electricity  by  transparent 
liquids  takes  place  in  a  different  manner  ~jp 
from  the  conduction  by  metals,  and  does  not  pgi 
affect  the  deduction,  which  can  be  shown 
mathematically  to  be  a  necessary  conse- 
quence of  the  theory,  namely,  that  all  good 
true  conductors  must  be  opaque  to  light.* 

Now  comes  the  question,  What  properties 
common  to  both  the  electric  and  optic 
ethers  can  we  observe  and  measure  so  as 
to  accurately  compare  them  ? 

The  first  property  we  shall  consider  is 
the  velocity  with  which  waves  travel  in 
each  case. 

*  It  must,  however,  be  confessed  that  gold,  silver 
and  platinum,  when  made  into  very  thin  plates,  are 
not  nearly  so  opaque  as  they  should  be  according  to 
the  theory. 

I 


114  FOUR   LECTURES 

I  mean  we  will  compare  the  velocity 
with  wliicli  light  waves  and  waves  of 
electro-magnetic  induction  move  in  air  and 
in  empty  space. 

In  both  cases  a  disturbance  is  propagated 
through  the  ether. 

If  a  candle  sends  light  to  the  eye,  the 
disturbance — the  wave,  that  is — travels 
over  the  space  between. 

Again,  if  an  electric  current  by  induction 
affects  a  magnet  at  a  distance,  the  strain, 
or  wave,  or  whatever  it  is,  travels  over  the 
space  between.  With  what  velocities  do 
the  disturbances  travel  in  each  case  ?  If 
these  velocities  can  be  measured,  and  if 
they  can  be  shown  to  be  the  same,  it  will 
be  a  very  strong  argument  for  considering 
that  the  electric  and  optic  ethers  are 
identical,  for  the  velocity  with  which  a 
wave  is  propagated  in  a  medium  is  a 
measure  of  its  density  and  its  elasticity. 

We  shall  then   consider  the  velocities 


ON   ELECTROSTATIC   INDUCTION.  115 

in  other  media.  Both  light  and  electro- 
magnetic induction  are  propagated  with  a 
different  velocity  in  glass  and  other  trans- 
parent solids  and  liquids  to  that  which  they 
have  in  air.  If  these  velocities  in  glass, 
&c.,  still  agree  with  each  other,  we  shall 
have  a  still  stronger  reason  for  supposing 
that  the  ethers  are  not  two,  but  one. 

Before  we  go  any  further  I  should  just 
like  to  remind  you  about  what  these  veloci- 
ties are  which  we  are  talking  of  measuring. 
That  of  light,  you  know,  is  about  185,000 
miles  per  second,  or  it  takes  about  eight 
minutes  to  pass  through  the  92,000,000 
miles  between  us  and  the  sun. 

The  velocity  of  light  has  been  measured 
directly  in  many  ways.  I  can  only  indicate 
one  of  them  here.  We  see  moving  bodies 
such  as  planets,  &c.,  at  any  instant  not 
in  the  position  which  they  are  at  that 
instant,  but  in  the  position  in  which  they 
were  when  the  light  left  them.  Thus,  if 

i  2 


116  FOUR   LECTURES 

a  planet  or  star  crossed  the  direct  line 
through  a  telescope  exactly  at  noon,  it 
would  not  be  seen  in  the  telescope  till 
some  time  after  noon.  The  length  of  this 
time  would  depend  on  the  distance  which 
the  light  had  to  travel. 

Now,  the  earth  goes  round  an  orbit  184 
million  miles  across,  and,  therefore,  the 
light  from  some  stars  has  to  travel  184 
million  miles  further  at  one  part  of  the 
year  than  at  six  months  later.  If  the 
instant  at  which  one  of  these  stars  is  seen 
to  cross  a  fixed  telescope  is  noted,  and  the 
same  observation  is  repeated  six  months 
later,  there  will  be  found  to  be  a  difference 
of  about  sixteen  minutes,  which  difference 
is  due  to  the  fact  that  at  one  time  the  dis- 
tance from  the  star  to  the  earth  is  longer  by 
some  184  million  miles  than  at  the  other. 
From  exact  measurements  of  this  time  the 
velocity  of  light  has  been  determined. 

Many   other   methods    of  measurement 


ON   ELECTROSTATIC   INDUCTION.          117 

have  been  used ;  in  one  of  which  the  time 
required  by  light  to  travel  fourteen  yards 
was  determined,  and  the  result  agreed  very 
well  with  the  one  where  the  distance  was 
the  diameter  of  the  earth's  orbit. 

The  results  obtained  by  different  ob- 
servers are  in  the  left  hand  upper  column 
of  the  table  (p.  118). 

The  velocity  of  electro-magnetic  induc- 
tion has  not  yet  been  measured  directly.* 
Probably  no  attempt  at  a  direct  measurement 
will  ever  be  made,  for  we  have  an  indirect 
method  of  computing  it,  which  is  susceptible 
of  far  greater  accuracy  than  is  ever  likely 
to  be  obtained  by  a  direct  measurement. 

A  comparison  of  electro-static  and  elec- 
tro-magnetic actions  is  the  basis  of  this 
measurement.  I  fear  the  process  is  too 

*  The  nearest  approach  to  a  direct  measurement  has 
been  made  by  Mr.  Eowland  and  Prof.  Helmholtz, 
Phil.  Mag.,  Sept.,  1876,  p.  233,  who  measured  the 
velocity  with  which  a  body  charged  statically  must 
be  moved  to  produce  a  certain  magnetic  effect. 


118 


FOUR  LECTURES 


VELOCITIES. 


MEDIUM. 

VELOCITY  OF  LIGHT. 

VELOCITY  OF  ELECTRO- 
MAGNETIC INDUCTION. 

Authority. 

Miles  per 
Second. 

Miles  pei- 
Second. 

Authority. 

Air  and  Vacuum  . 

Pizeau  . 
Astronomi- 
cal obser- 
vations  . 
Foucault    . 
Cornu   .     . 

195,100 
1 
J.108,800 

185,300 
187,100 

193,000 
187,900 

178,900 
175,200 
183,100 

185,300 

Weber 
Rowland  & 
}  Helmholtz 
Maxwell 
Thomson 
M'Kichan 
f  Ayrton  & 
\,     Perry 

Mean     .    .. 

189,700 
186 

183,900 
000 

Mean 

Double        extra  j 
dense        flint  |> 
glass     -     .     .J 

)  Gordon  ; 

/                \ 

106,400 

104,600 

\ 
Gordon 

Extra  dense  flint  \^ 
glass      .     .     .J 

110,900 

106,400 

Light  flint  glass  . 

123,100 

107,300 

Sard  crown  glass 

116,800 

105,500 

Paraflin    .     .     J 

Gladstone 
and 
Maxwell     . 

1  130,800 

131,700 

'late  glass  .     .     . 

1    Text 
!  Books  1 

121,000 

103,000 

Sulphur  .... 

89,000 

115,000 

Bisulphide       of") 
Carbon      .     .  / 

115,000 

139,000 

ON   ELECTEOSTATIC   INDUCTION.          119 

purely  mathematical  to  enable  me  to 
explain  it  in  detail,  but  I  can  indicate  the 
principle  of  it.  If  the  same  thing,  such  as 
the  resistance  which  a  wire  offers  to  the 
passage  of  electricity,  be  measured  both 
electro-statically  and  electro-magnetically, 
the  numbers  obtained  for  the  result  will 
not  be  the  same. 

The  difference  is  caused  by  one  set  of 
measurements  being  based  on  the  con- 
sideration of  electricity  at  rest,  and  the 
other  on  the  consideration  of  electricity  in 
motion.  Mathematics  tell  us  that  the 
ratio  of  the  results  is  a  motion  or  a  velocity, 
and  that  this  velocity  is  the  velocity  of 
electro -magnetic  induction ;  therefore,  from 
such  measurements  this  velocity  can  be 
calculated.  A  comparison  of  the  two  upper 
columns  of  this  table  shows  a  very  close 
agreement  between  the  velocities  of  electro- 
magnetic induction  and  of  light.  You 
notice  that  the  difference  of  the  means  of 


120  FOUR   LECTURES 

the  two  columns  is  less  than  the  accidental 
differences  between  numbers  in  the  same 
column.  Thus  we  have  seen  that  in  air 
and  vacuum  the  velocities  of  light  and  electro- 
magnetic induction  are  sensibly  equal. 

Now  we  will  consider  the  case  of  glass 
and  other  transparent  insulators. 


In  these  light  does  not  travel  with  the 
same  velocity  as  in  air. 

How  do  we  determine  the  velocity  of 
light  in  glass?  "We  do  not  determine  it 
directly,  but  measure  the  difference  of  velo- 
city in  air  and  glass.  Let  a  b  (Fig.  34)  be 
the  front  of  a  wave  of  light  travelling  along 
in  the  direction  of  the  arrow.  Let  it  fall  on 
a  piece  of  glass  M  M,  placed  diagonally 


ON   ELECTROSTATIC   INDUCTION.          121 


* 


to  the  direction  of  the  ray,  or  arrow. 
When  the  wave  gets  to  the  position  of  V 
the  lower  part  of  begins  to  enter  the  glass, 
the  upper  part  V  is  still  in  the  air.  But 
as  light  travels  more  slowly  in  glass  than 
in  air,  by  the  time  the  top  V  has  got  to 
the  glass  at  V  moving  in  air,  the  lower 
part  a  moving  in  glass  has  not  travelled 
so  far  as  the  upper  part,  and  has  only  got 
to  a".  The  lower  part  of  the  wave  is 
retarded  or  dragged  back  behind  the  upper 
part  and  the  wave,  and,  consequently,  the 
direction  of  the  ray  is  twisted  round,  so  as 
to  make  an  angle  with  its  former  direction. 

Suppose  two  people  are  pushing  a  two- 
wheeled  cart  along  by  turning  the  wheels. 
If  one  turns  his  wheel  faster  than  the 
other  does,  the  direction  in  which  the  cart 
travels  will  be  changed. 

Now,  if  we  measure  the  angle  through 
which  the  cart  turns,  we  can  calculate  the 
difference  in  the  speeds  at  which  the  two 


122  FOUR   LECTURES 

wheels  are  moving.  Similarly,  if  we 
measure  the  angle  through  which  our  ray  of 
light  is  turned,  we  can  calculate  the  differ- 
ence of  the  velocities  with  which  the  part 
in  air  and  the  part  in  glass  were  moving. 

The  lower  left-hand  column  of  the  table 
(page  ]  18)  shows  the  results  of  the  calcu- 
lations for  the  velocities  of  light. 

The  velocities  of  the  electro-magnetic 
induction  are  calculated  from  the  velocity 
in  air. 

Theory  shows  that  this  calculation  can 
be  made  when  we  know  the  specific 
inductive  capacities. 

The  greater  the  specific  inductive 
capacities  the  slower  the  electro -magnetic 
induction  travels. 

The  lower  right-hand  column  of  the 
table  (page  118)  shows  the  results  of  the 
calculations  for  the  velocities  of  electro- 
magnetic induction.* 

*  The  velocities  in  the  lower  half  of  the  table  are 


ON   ELECTROSTATIC   INDUCTION. 


128 


You  see  that  in  certain  dielectrics  there 
is  a  very  close  agreement  indeed,  notably 
in  paraffin  and  in  some  of  the  denser 

calculated  from  the  refractive  indices  and  specific  in- 
ductive capacities  given  in  the  following  table  : — 


Dielectric. 

N/K 

Nearest 
value 
of/*. 

Ray  for  which  /«,  is 
nearest. 

Double  extra  dense 
flint  glass  

1-7783 

1-7460 

Band  in  extreme 

Extra  dense  flint  .  , 

1-7474 

1-6757 

nesium     spark 

Light  flint  

1-7343 

1-5113 

Hard  crown 

1-7629 

1-5920 

Plate  glass   

1-8009 

1-543 

Paraffin  

1-4119 

T422 

Rays  of   infinite 

wave  length. 

Sulphur  

1-6060 

2-115 

Bisulphide  of  car- 
bon 

1-3456 

1.6114 

Here  K  is  the  specific  inductive  capacity,  and 
stands  for  the  square  root  of  K.    /w,  is  the  refractive 
index. 

Maxwell's  theory  gives  that  the  velocity  of  electric 
induction  in  any  dielectric  is  to  the  velocity  in  air 


124  FOUR   LECTURES 

glasses;  in  others  there  is  a  very  wide 
difference. 

I  am  hoping  shortly  to  make  some 
further  experiments,  both  on  sulphur  and 
on  bisulphide  of  carbon.  I  am  not  certain 
of  the  accuracy  of  my  determination  of  the 
specific  inductive  capacity  of  bisulphide,  as 
liquids  present  special  difficulties ;  while  as 
to  sulphur,  it  has  so  many  different  forms 
— yellow  crystal,  red  powder,  black  plastic 

inversely  as  the  square  root  of  the  specific  inductive 
capacity.  To  determine  the  velocity  of  electric  in- 
duction in  any  dielectric  186,000,  the  adopted  mean 
velocity  in  air,  is  divided  by  the  number  under  the 
heading  \/7£.  in  this  table. 

"We  also  know  that  the  velocity  of  light  in  any 
medium  is  inversely  as  the  refractive  index  ^  Hence 
the  velocity  of  light  in  each  dielectric  is  found  by 
dividing  186,000  by  the  number  under  the  heading  /*. 

The  values  of  \/K  are  from  the  author's  determina- 
tions of  specific  inductive  capacity  (page  90).  //,  was 
determined  by  the  author  for  the  first  four  glasses ;  for 
paraffin  the  value  given  is  that  calculated  by  Maxwell 
from  Gladstone's  experiments.  For  the  other  sub- 
stances, where  there  is  a  wide  difference,  the  values 
of  /A  are  taken  from  the  text-books. 


ON   ELECTROSTATIC   INDUCTION.          125 

wax — that  it  is  possible  that  the  discrepancy 
may  be  partly  accounted  for  by  supposing 
that  there  was  a  difference  in  the  physical 
state  of  the  sulphur  when  the  electrical 
and  optical  experiments  were  made. 

At  present,  however,  I  think  we  may 
fairly  say  that  in  some  dielectrics  the  velo- 
city of  electro -magnetic  induction  is  nearly 
equal  to  the  velocity  of  light.  That  there 
is  almost  always  a  small  difference,  and 
that  sometimes  there  is  a  very  large 
difference. 

That  it  is  quite  possible  that  the  relation 
which  we  have  spoken  of  between  electric 
induction  and  light  exists,  namely,  that 
they  are  disturbances  of  the  same  ether; 
but  that  there  is  some  unknown  disturbing 
cause  affecting  the  electric  induction,  and 
that  in  some  dielectrics  the  disturbing 
cause  is  very  small,  but  that  it  is  in  others 
large  enough  to  cause  a  very  large  difference 
between  the  velocities  of  light  and  the 


126  FOUR   LECTUEES 

calculated  velocities  of  electro-magnetic 
induction. 

I  am  hoping  some  day  to  compare  the 
electro-static  inductions  along  and  across 
the  axis  of  the  same  crystal. 

I  hope  that  the  disturbing  cause,  what- 
ever it  may  be,  may  affect  the  inductions 
equally  in  both  cases. 

We  know  that  the  velocity  of  light  in  a 
crystal  is  different  along  and  across  the 
axis,  and  if  then  the  ratio  of  the  two  light 
velocities  were  the  same  as  the  ratio  of  the 
two  electro-magnetic  velocities,  we  should 
have  a  confirmation  of  the  theory,  indepen- 
dent of  any  knowledge  of  the  nature  of 
the  disturbing  cause. 

What  this  disturbing  cause  may  be  we 
do  not  know.  Perhaps  some  future 
investigation  may  explain  its  real  nature. 

I  am  now  going  to  attempt  to  show  you 
a  few  experiments  to  illustrate  other 
connections  between  electricity  and  light, 


ON   ELECTEOSTATIC   INDUCTION.          127 

experiments  in  which  electricity  acts  on 
light  and  vice  versa.  In  fact,  without  too 
bold  a  hypothesis,  we  may  call  them 
experiments  where,  on  the  ether  in  certain 
bodies  being  disturbed  by  electricity,  the 
disturbance  is  seen  in  their  changed  action 
on  light,  and  where,  when  it  is  disturbed 
by  light,  their  action  on  electricity  is  ri 
altered. 

The  first  action  which  I  will  show  you  rv  • 
is  the  electro-magnetic  action  discovered  CD 
by  Faraday. 

Faraday  found  that  if  an  electric  current 
were  made  to  circulate  round  and  round 
a  cylindrical  ray  of  light,  that,  in  certain 
media,  the  ray  would  be  twisted,  so  that  a 
line  drawn  along  the  outside  of  the  ray 
would  no  longer  be  straight,  but  would  be 
twisted  spirally  like  the  rifling  of  a  gun. 

But  how  are  we  to  draw  this  line,  and 
how  see  the  twist  of  it  ? 

You   know  that   in  ordinary   light   the 


128  FOUR   LECTURES 

vibrations  take  place  in  every  possible 
direction  at  right  angles  to  the  ray. 
Crystals  of  Iceland  spar,  cut  and  arranged 
in  a  particular  way,  called  "  Mcols'  prisms," 
have,  however,  the  power  of  compelling  all 
the  vibrations  to  take  place  in  one  particular 
plane,  or  of  polarizing  the  light,  as  it  is 
called.  In  fact,  when  the  light  is  plane- 
polarized,  we  have  a  flat  ray  of  light 
instead  of  a  cylindrical  one.  Let  us  now 
pass  light  horizontally  through  a  Mcols5 
prism,  so  as  to  polarize  it,  let  us  say,  in  a 
horizontal  plane. 

We  shall  now  have  extinguished  all 
vibrations,  except  those  in  one  plane,  say 
the  horizontal  plane. 

Let  us  now  put  a  second  prism,  with  its 
polarizing  plane  vertical ;  it  will  have  the 
power  of  extinguishing  all  horizontal 
vibrations,  and  it  will,  therefore,  entirely 
extinguish  all  the  light  which  has  come 
from  the  first  prism,  as  you  see.  But  if 


ON   ELECTEOSTATIC   INDUCTION.          129 

the  plane  of  the  light  is  twisted  by  any  means 
between  the  two  prisms,  it  will  no  longer  fall 
with  its  vibrations  horizontal  on  the  second 
prism,  but  will  be  partly  allowed  to  pass,  more 
and  more  of  it  being  admitted  as  the  plane 
is  twisted  more  and  more  nearly  vertical. 
Here,  between  the  prisms  (Fig.  35),  is  a 


tube  with  glass  ends,  filled  with  bisulphide 
of  carbon,  and  here  is  a  coil  of  wire  of  1028 
turns,  carrying  an  electric  current  round 
and  round  the  tube.  The  light  passes  along 
the  tube.  The  prisms  are  set  to  extinction. 
I  turn  on  the  electric  current  of  ten  Groves 
cells,  and  you  see  the  re-appearance  of  the 
light  on  the  screen  shows  that  the  plane  of 
polarization  has  been  twisted.  I  have  to 
turn  the  prism  through  about  14°  to  again 

K 


130  FOUR   LECTURES 

extinguish  it.  The  number  of  degrees  which 
the  prism  has  to  be  turned  of  course  show 
the  amount  of  twist  that  has  been  given  to 
the  light.  On  reversing  the  current  the  light 
re-appears,  and  to  extinguish  it  we  have  to 
turn  the  prism  as  far  to  the  right  of  the 
zero  as  we  previously  turned  it  to  the  left.* 
Now,  as  many  of  you  will  know,  a 
spiral  current  acts  just  like  a  magnetized 
bar  would  if  placed  in  the  spiral.  In  fact, 
if  light  is  sent  from  pole  to  pole  of  a 
magnet,  the  same  effect  is  produced  on  it 
as  that  which  we  have  just  seen.  In  1877  a 
paper  of  mine  was  published  in  the  "  Phi- 
losophical Transactions, "t  containing  mea- 
surements of  the  amount  of  twist  which  a 
unit  magnetic  force  would  give  to  a  ray  of 
light,  and  in  the  "  Comptes  Eendus  "  for 

*  See  Faraday,  "Experimental  Kesearches,"  §  2146, 
voL  iii. ;  and  Verdet,  GEuvres,  T.  i.,  Notes  et  Memoires. 

t  Vol.  167,  p.  1,  "  On  the  Determination  of  Yerdet's 
Constant  or  Absolute  Units." 


ON    ELECTROSTATIC   INDUCTION.  131 

1878,*  M.  H.  Becquerel  pointed  out  that 
from  these  results  we  can  calculate  what 
would  be  the  effect  of  the  earth's  magnetism 
on  light  in  certain  media. 

If  a  canal  one  mile  long  were  dug  from 
north  to  south  near  Kew,  and  filled  with 
bisulphide  of  carbon,  a  ray  of  green 
polarized  light  entering  at  one  end,  would, 
by  the  action  of  the  earth's  magnetism, 
have  its  plane  of  polarization  twisted  just 
50°.  There  are  slight  differences  in  the 
action  on  different  coloured  lights.  My 
measurements  were  made  on  the  green  light 
of  burning  thallium.  If  the  canal  had  been 
full  of  distilled  water  the  twist  would  have 
been  about  7-|°. 

The  explanation  of  this  phenomenon  is 
still    exceedingly    obscure.      We    as    yet 
know  so  little  about  the  molecular  structure 
of  bodies  that  there  are  very  many  gaps  in 
the  chain  of  reasoning,  that  must  either 
*  T.  IxxxvL  p.  1077. 
K  2 


132  FOUR   LECTUEES 

still  be  left  empty  or  filled  in  with 
provisional  hypotheses,  that  is  guesses. 

What  we  do  know  on  this  subject  may 
be  briefly  summed  up  as  follows  :  In  the 
disturbance  which  we  call  light,  whatever 
its  true  nature  may  be,  we  know  that  there  is 
something  like  a  rotation  round  an  axis  going 
on,  which  axis  is  the  direction  of  the  ray. 

When  magnetic  forces  act  on  a  medium, 
Professor  Maxwell  has  shown  that  there  is 
always  something  like  a  rotation  round  an 
axis  going  on,  which  axis  is  the  line  of 
force.  But  here  the  resemblance  stops. 
There  is  nothing  in  the  magnetic  pheno- 
menon which  corresponds  to  the  wave 
length  and  wave  propagation  in  the  optical 
phenomena. 

As  to  the  nature  of  the  rotation  ac- 
companying the  magnetic  forces,  we  know 
that  it  exists,  and  we  know  that  it  is  not 
the  rotation  of  any  sensible  portions  of  the 
medium  as  a  whole. 


ON   ELECTROSTATIC    INDUCTION.  133 

Professor  Clerk  Maxwell  suggests  that 
it  may  be  a  rotation  of"  molecular  vortices," 
that  is,  that  every  part  of  the  magnetized 
medium  may  be  filled  with  little  whirlpools 
exceedingly  minute.  These  whirlpools 
may  be  motions  of  ultimate  particles  of 
matter,  or  may  be  motions  of  the  ether  in 
it,  or  possibly  in  this  region  of  "very- 
smallness  "  the  ether  and  the  matter  may 
be  one.  Now,  as  minute  eddies  in  a 
stream  whirl  chips  round  but  do  not 
affect  a  large  boat,  so  these  whirlpools, 
while  they  cannot  affect  the  sensible 
motions  of  bodies,  may  be  able  to  influence 
greatly  the  minute  vibrations  which  are 
the  propagation  of  light. 

The  next  experiment  I  have  to  tell  you 
about  is  on  the  action  of  electro- static  in- 
duction upon  light.  You  know  that  if 
light  is  sent  through  a  crystal  it  is  acted 
on  in  a  way  different  to  the  action  which 
occurs  when  it  passes  through  a  homo- 


134  FOUR   LECTURES 

geneous  medium.  In  particular,  if  light  is 
plane  polarized  before  it  enters  the  crystal 
— that  is,  if  its  vibrations  are  all  in  one 
plane — then,  after  it  emerges  from  the 
crystal,  some  of  the  vibrations  will  be  cir- 
cular, and  in  no  position  of  the  second 
Mcols  can  the  light  be  extinguished. 
In  Nov.  1875,  Dr.  Kerr*  announced 


that  when  glass  is  subjected  to  an  intense 
electro- static  strain  it  acquires  the  same 
action  on  light  as  a  crystal  has.  In  fact, 
that  the  electric  strain  so  rearranges  the 
molecules  of  the  glass  that  they  act  on  light 
as  if  they  were  the  naturally-arranged 
molecules  of  a  crystal.  Here  is  the  method 
*  "Phil.  Mag.,"  1875,  part  ii.  p.  337. 


ON   ELECTROSTATIC   INDUCTION.  135 

he  used  (Fig.  36),  but  the  effect,  though 
quite  decided,  is  so  minute,  and  the  con- 
ditions of  success  are  so  delicate,  that  I 
have  not  much  hope  of  showing  the  actual 
experiment  to  you. 

Here  is  a  piece  of  thick  plate* glass,  about 
eight  inches  long  and  two  wide  having  two 
holes  drilled  in  it  from  the  ends,  so  that  they 
come  within  about  3-1 6th  of  an  inch  of  each 
other.  (Fig.  37).  Into  these  holes  wires 

FIG  .  37 


are  cemented.  These  wires  are  connected 
to  the  secondary  poles  of  the  large  induc- 
tion coil,  which  we  used  last  lecture  (see 
Fig.  23),  only  now  it  is  used  with  its  full 
power,  with  ten  quart-sized  Grove  cells,  and 
its  own  vibrating  break.  When  the  coil  is 
worked,  the  tension  across  the  3-1 6th  of  an 
inch  of  glass  is,  of  course,  equal  to  the  ten- 
sion across  the  air  between  the  discharging 


136  FOUR   LECTURES 

poles  where  the  sparks  are  passing.  By 
commencing  with  the  poles  close  together, 
and  then  gradually  drawing  them  apart, 
the  tension  across  the  glass  can  be  increased 
as  we  please.  The  Nicols  prisms  and  a 
lens  for  focussing  are  arranged,  as  shown 
in  Fig.  36.  An  alum  cell  is  attached  to 
the  electric  lamp  to  intercept  the  heat 
rays.  Now,  we  turn  the  Nicols,  so  as  to 
darken  the  screen,  start  the  coil,  and  gra- 
dually draw  the  discharging  poles  apart. 
When  we  get  to  a  tension  equal  to  about 
seven  inches  of  air  you  see  the  patch  of 
white  light*  appearing  on  the  screen.  (This 
was  clearly  seen  by  a  large  audience.) 

*  In  rehearsing  this  experiment  the  day  before,  Mr. 
Cottrell,  Mr.  Yalter  (the  second  assistant),  and  myself 
only  being  present,  the  strain  was  accidentally  allowed 
to  become  too  great,  and  the  glass  was  perforated. 
Immediately  before  perforation  some  extraordinary 
effects  were  seen  on  the  screen.  First  appeared  a 
patch  of  orange-brown  light  about  six  or  seven  inches 
diameter.  This  at  once  resolved  itself  into  a  series  of  four 


ON   ELECTROSTATIC   INDUCTION.  137 

Dr.  Kerr  found  that  the  maximum  .effect 
was  produced  when  the  prisms  were  so  set 
that  the  line  of  electric  strain  was  at  45°  to 
the  direction  of  optical  vibration.  (Fig. 
38.)  As  the  prisms  are  turned  away  from 
that  position  the  effect  gets  less  and  less, 
till,  when  the  direction  of  vibration  is  either 

or  five  irregular,  concentric  rings,  dark  and  orange-brown, 
the  outer  one  being,  perhaps,  fourteen  inches  diameter. 
In  about  two  seconds  more  these  vanished  and  were 
succeeded  by  a  huge  black  cross  about  three  feet  across, 
seen  on  a  faintly  luminous  ground.  The  arms  of  the 
cross  were  along  the  planes  of  polarization,  and,  there- 
fore (the  experiment  being  arranged  according  to  Dr. 
Kerr's  directions),  were  at  45°  to  the  line  of  stress. 
The  glass  then  gave  way,  and  all  the  phenomena  dis- 
appeared except  the  extreme  ends  of  the  cross,  and  the 
discharge  through  the  hole,  where  the  glass  had  been 
perforated,  was  alone  seen.  I  have  since  made  nume- 
rous attempts  to  repeat  this  effect,  but  have  not  suc- 
ceeded in  doing  so,  though  I  have  perforated  many 
valuable  glasses.  In  this '  particular  case  the  glass 
happened  to  break  slowly.  In  all  the  repetitions  of 
the  experiment  the  glass  has  broken  suddenly,  and 
there  has  been  no  time  for  the  new  effects  to  occur. 
Proc.  Roy.  Soc.,  Feb.  13,  1879. 


138  FOUR   LECTURES 

along  or  perpendicular  to  the  line  of  strain, 
there  is  no  effect  at  all. 

,    r.c.3.    ,,- 

.  •  —  — J^. — j  ,    .       , 


a  a  line  of  electric  strain. 
b  b,  b  b'  direction  of  optical  vibrations. 
Ray  of  light  perpendicular  to  plane 
of  paper. 

We  observe  that  in  this  experiment  there 
is  no  rotation  of  plane- polarized  light  as  in 
the  last  one,  for  no  rotation  of  the  Nicols  will 
extinguish  the  light.  It  is  found  that  the 
light  after  emerging  from  the  strained 
glass,  is  no  longer  plane-polarized,  but  that 
its  vibrations  are  circular. 

I  have  still  one  more  experiment  to  show 
you. 

Here  is  a  metal  called  selenium.  It  con- 
ducts electricity,  but  very  badly — that  is, 
it  offers  a  great  but  not  an  infinite  resist- 
ance to  the  straining  force. 

But  it  has  this   extraordinary  property. 


ON   ELECTROSTATIC   INDUCTION.  139 

It  conducts  much  better  in  the  light  than 
in  the  dark.  The  light  vibrations  actually 
seem  to  shake  its  molecules,  and  help  them 
to  yield  to  the  electric  strain. 

I  will  try  to  show  you  this  experimen- 


Here  you  see  the  arrangement.  (Fig.  39,) 


o 

CD 


Here  is  a  delicate  reflecting  galvanometer 
of  high  resistance.     On  pressing  down  the  ;  / 

contact  key  the  current  from  a  ten-cell  j~~) 
Leclanche  battery  flows  through  the  gal- 
vanometer, and  through  a  piece  of  selenium 
enclosed  in  a  light-tight  box,  and  you  see 
the  deflection  of  the  needle  moves  the  spot 
of  light  over  about  ten  divisions  of  the  scale. 
Now,  we  light  this  piece  of  magnesium 
wire,  and  draw  up  the  sliding  front  of  the 


140  FOUR   LECTURES 

box.  The  deflection  is  at  once  doubled, 
showing  that  in  the  light  this  selenium 
conducts  about  twice  as  well  as  in  the  dark. 

Prof.  W.  G.  Adams  has  made  many  ex- 
periments on  this  subject,*  and  he  found 
that  light  can  actually  produce  a  current 
of  electricity,  and  not  merely  aid  its 
passage. 

Here  f  is  a  box  containing  a  piece  of 
selenium  whose  ends  are  connected  to  the 
galvanometer.  I  open  the  box  and  admit 
the  light  of  the  electric  lamp  and  there 
is  a  deflection  of  the  galvanometer. 

The  electricity  of  a  battery  has  been 
converted  into  light  in  the  lamp,  and  that 
very  same  light  is  again  converted  into 
electricity  in  the  selenium. 


In  these  four  lectures  we  have  considered 
some  few  phenomena  which  we  can  explain, 

*  Proc.  Eoy.  Soc.,  1876,  xxiv.  p.  163. 

t  This  last  experiment  was  omitted  for  want  of  time. 


ON   ELECTROSTATIC   INDUCTION.  141 

and  a  great  many  which  we  cannot.  Most  of 
the  experimental  facts  stand  as  yet  alone 
and  disjointed.  Many  lines  of  reasoning  and 
research  open  out  a  little  way,  and  then 
are  lost  in  the  darkness,  or,  rather,  let  us 
say  in  the  brightness  through  which,  as 
yet,  human  sight  cannot  pierce. 

No  doubt  the  day  will  come  when  all 
these  difficult  ways  will  be  clear  and 
trodden  paths,  when  all  these  disjointed 
facts  will  be  seen  to  be  parts  of  one  true, 
harmonious,  and  perfect  whole. 


THE    END. 


INDEX. 


Adams,  Prof.  W.  G.,  F.K.S.,  on  selenium,  140. 

Apps,  82. 

Ayrton  and  Perry  on  specific   inductive  capacities  of  gases, 

93,  99. 

Ayrton  and  Perry,  velocity  of  electro-magnetic  induction,  118. 
Balance,  the  Induction,  74. 
Barclay  and  Gibson  on  specific  inductive  capacity  of  paraffin, 

91. 
Becquerel,  H.,  action  of  earth's  magnetism  on  polarized  light, 

131. 

Break,  rapid,  80. 
Callipers,  77. 
Capacity,  specific  inductive,  see   specific  inductive    capacity, 

49  et  seq. 

Closed  condenser,  Ayrton  and  Perry,  96. 
Coil,  induction,  with  rapid  break,  79. 
Conductors  and  insulators,  8. 
Conductors  opaque  to  light,  113. 
Cornu,  velocity  of  light,  118. 
Curved  discharge  of  Holtz  machine,  45. 
Curved  lines  of  induction,  42. 

De  la  Rue  on  lateral  pressure  in  a  vacuum  tube,  48. 
Determination  of  specific  inductive  capacity  before  audience, 

86. 

Discharge  by  finger,  10. 

Earth's  magnetism,  action  of,  on  polarized  light,  131. 
Electricity  and  light,  relations  between,  100. 
Electrification  by  induction,  9. 

Electro-magnetic  theory  of  light,  Clerk  Maxwell,  102. 
Electroscope,  gold-leaf,  13. 
Electro-static  action  on  polarized  light,  133. 
Energy,  potential  and  kinetic,  in  ether,  107. 
Equal  quantities  of  two  electricities  always  produced,  22. 
Equality  of  two  kinds  of  induced  electricity,  12. 
Ether,  hypothesis  of,  106. 


144  INDEX. 


Faraday,  Electro-magnetic  rotation  of  plane  of  polarization  of 
light,  127. 

,  Extract  from  Exp.  Res.  of,  1. 

on  induction  in  curved  lines,  40. 

on  mutual  attraction  of  lines  of  force,  47. 

on  specific  inductive  capacity,  53. 

Fizeau,  Velocity  of  light,  118. 

Force  between  electrified  bodies,  7. 

Foucault,  Velocity  of  light,  118. 

Fresnel,  107. 

Gases,  specific  inductive  capacity  of,  92,  99. 

Gibson  and  Barclay  on  specific  inductive  capacity  of  paraffin, 

91. 

Gladstone  and  Maxwell,  Velocity  of  light  in  paraffin,  118. 
Glass,  electrification  of,  by  friction,  2. 
Gordon,  Absolute  measurement  of  electro-magnetic  rotation 

of  plane  of  polarized  light,  130. 
— ,  Measurements  of  specific  inductive  capacity,  63. 

Callipers,  77. 

Conditions  required,  63. 

Diagram  of  method,  66. 

Formula  of  calculation,  69  (note). 

Induction  balance,  74. 

Mechanical  slide,  76. 

Eapid  reversals,  64,  82. 

Results,  90. 

Zero  method,  64,  88. 

,   Velocity   of   electro-magnetic   induction   in   various 

dielectrics,  118,  122  (note). 

-,  Velocity  of  light  in  glass,  118,  122  (note). 


Green,  107. 

Heating  glass  allows  current  to  pass  and  stops  induction,  20. 
Helmholtz   and   Rowland,  Velocity   of  electro-magnetic   in- 
duction, 117  (note),  118. 
Holtz  machine,  curved  discharge  of,  45. 
Hopkinson's     experiment,    showing    that    tapping     hastens 

return  of  residual  charge,  34. 
Huygens,  107. 

Induction,  cannot  pass  through  a  metal  screen,  41. 
Curved  lines  of,  42. 
Electrification  by,  9. 
Induction,  precedes  discharge,  44. 
Propagation  of,  39. 


INDEX.  145 


Induction  balance,  the,  74. 

Insulators  and  conductors,  8. 

Kerr,    Eev.    J.,    LL.D.,    Direction  of    vibration    in    his    ex- 
periments, 137. 

,  Discovery  of  electro- static  action  on  polarized  light, 

134. 

,  Extension  of  his  experiments,  136  (note). 

Kieser,  78. 

Kinetic  energy,  explanation  of  term,  107. 

Lateral  pressure  accompanies  tension  force  of  induction,  47. 

Leyden  jar,  25. 

Discharge  of,  28. 
Theory  of,  27. 

Light  and  electricity,  relations  between,  100. 

Light,  Maxwell's  electro-magnetic  theory  of,  102. 

Machinery  of  induction,  what  is  the  nature  of  it  ?  15. 

Maxwell,  J.  Clerk,  Electro- magnetic  theory  of  light,  102. 

and  Gladstone,  Velocity  of  light  in  paraffin,  118. 

on  lateral  pressure  accompanying  induction,  47. 

,  Velocity  of  electro-magnetic  induction,  118. 

Mechanical  illustration,  of  induction,  16. 

Of  residual  charge,  30. 

M'Kichan,  Velocity  of  electro-magnetic  induction,  118. 

Molecular  vortices,  133. 

Opacity  of  conductors  to  light,  113. 

Open  condenser,  Ayrton  and  Perry,  94. 

Perry  and  Ayrton  on  specific   inductive  capacities  of  gases, 
93,  99. 

,  Velocity  of  electro-magnetic  induction,  118. 

Plan  of  laboratory,  84. 

Polarized  light,  electro- magnetic  rotation  of  plane  of,  127. 

Absolute  measures  of  the  rotation,  by  Lecturer,  130. 

Potential  energy,  explanation  of  term,  107. 

Propagation  of  induction,  39. 

Quadrant  electrometer,  Thomson's,  67  ;  explained,  35. 

Eapid  break,  80. 

Eemoval  of  far  end  of  conductor,  19. 

Residual  charge,  29. 

Mechanical  illustration  of,  30. 

Rotation  accompanies  magnetic  force,  132. 

Rowland   and   Helmholtz,   Velocity   of    electro-magnetic   in- 
duction, 117  (note),  118. 

Screen  of  metal,  induction  cannot  pass  through  a,  41. 


146  INDEX. 


Sealing-wax,  electrification  of,  by  friction,  4. 
Secondary  reversing  engine,  83. 
Selenium,  light  increases  conductivity  of,  139. 
Light  can  produce  a  current  in,  140. 
Slide,  mechanical,  of  induction  balance,  76. 
Specific  inductive  capacity,  49. 

Definition  of,  50. 
Faraday's  apparatus,  54. 

Experiments  on,  53. 
Results,  60. 
Importance  of  accurate  measure- 

ments of,  61. 
New  measurements  of,  by  Lecturer, 

see  Gordon's  measurements,  63. 
Table  of,  90. 
Specific  inductive  capacity  of  gases,  92,  99. 

Ayrton  and  Perry  on,  93,  99. 
Faiaday  on,  92. 
Speed  of  signalling,  62. 
Strain,  explanation  of  word,  106. 
Strength,  electric,  of  air,  48. 
Sulphuric  acid  as  conductors  of  Leyden  jar,  37. 
Table,  comparing  ^/K  and  /*,  123  (note). 
.  Of  specific  inductive  capacities,  90. 

of  gases,  99. 

Velocities  of  light  and  electro-magnetic  induction,  118. 
Tension  force  of  induction  is  accompanied  by  lateral  pressure, 

47. 

Thomson,  Sir  Wm.,  on  electric  strength  of  air,  48. 
-  ,  Quadrant  electrometer,  35,  67. 
--  ,  Velocity  of  electro-magnetic  induction,  118. 
Transverse  vibrations,  110. 

Velocity  of   electro-magnetic  induction   in  air  and  vacuum, 
117,  118. 

In  other  media,  118,  122. 
Of  light  in  air  and  vacuum,  115,  118. 

In  other  media,  118,  120. 
Verdet,  130. 

Vibrations  of  induction  perpendicular  to  line  of  force,  111. 
Vortices,  molecular,  133. 
Wave  of  induction,  diagram  of,  112. 
Weber,  Velocity  of  electro  -magnetic  induction,  118. 
Young,  107. 
Zero  method,  64,  88. 

OF 


J117SBSIT7 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


OCT 


ftPT 


1Q3R 


JJNIVERSITY  OF  CALIFORNIA  LIBRARY 


I 


